A Nice Math Problem

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  • Опубліковано 22 лис 2024

КОМЕНТАРІ • 130

  • @OndrejPopp
    @OndrejPopp Рік тому +63

    When you factor 1680 you get 2^4×3×5×7 Now to write this as a product of four terms that differ by one, you can find the solution almost immediately : 5×(2×3)×7×2^3 = 5×6×7×8. So, x+6 = 8 => x=2

    • @ilyaf1040
      @ilyaf1040 Рік тому +4

      Именно так и решают такие задачи в России. This is exactly how such problems are solved in Russia.

    • @samueldeandrade8535
      @samueldeandrade8535 11 місяців тому

      Don't tell me he solved differently ...

    • @samueldeandrade8535
      @samueldeandrade8535 11 місяців тому +2

      I can't believe he went to polynomials.

    • @viswanathm.s.2424
      @viswanathm.s.2424 11 місяців тому

      ​@@ilyaf1040hu hu too😮😮😮😮😮😮😮😮😮😮😮😮😮

    • @vincentgary4786
      @vincentgary4786 11 місяців тому +6

      De cette façon, vous avez trouvé que x=2 est une solution. Mais est-ce la seule?

  • @goldfing5898
    @goldfing5898 Рік тому +22

    If you are certain that x is a natural number, you can also shorten the process by guessing:
    (x + 6)(x + 5)(x + 4)(x + 3) = 1680
    Choose the arithmetic mean of the four factors for a new variable: t = x + 4.5
    And replace all factors by t for a first guess: t * t * t * t = 1680
    t^4 = 1680
    Take the square root. Since 40^2 = 1600 and 41^2 = 1681, we get t^2 = 41 approximately.
    Take the square root once again. Since 6^2 = 36 and 7^2 = 49, we get t = 6.5 approximately.
    Altogether, we can estimate t = x + 4.5 = 6.5.
    Subtract 4.5 to estimate x = 2. Then do the proof:
    x + 3 = 5
    x + 4 = 6
    x + 5 = 7
    x + 6 = 8
    And the product is
    5 * 6 * 7 * 8
    = 30 * 7 * 8
    = 210 * 8
    = 1680.

  • @samwong3123
    @samwong3123 11 місяців тому +16

    By inspection, 1680 is a product of 4 consecutive numbers. It is a lot quicker to work out 4th root of 1680 is between 6 and 7. Hence 1680 is = 5x6x7x8 and x=2 (dont have to deal with factorial of negative number as it is not defined)

  • @larrybrautigam7734
    @larrybrautigam7734 11 місяців тому +6

    Square root of 1680 is about 40. 1680/40 = 42. 42 = 6x7, so easy light bulb moment that 40 = 5x8.
    Good problem. Looks at first waay difficult, then...
    Satisfying solve.

  • @raphaelnjabulodlamini9412
    @raphaelnjabulodlamini9412 10 місяців тому +3

    As a math enthusiast who ended primary school grade 7 level and trying to learn math, i think her elaborate steps are helpful - i love them....❤

  • @sudhangshubhattacharya4991
    @sudhangshubhattacharya4991 5 місяців тому

    Veri wisely catch up the problem

  • @Seongjam
    @Seongjam Рік тому +7

    Really fun to watch!

  • @88kgs
    @88kgs Рік тому +4

    Sir you have a very good handwriting, and you explained the problem in detail and very patiently 😊

  • @lornawilliams550
    @lornawilliams550 Рік тому +2

    Nice problem. Clear explanation

  • @FrancisSolvang
    @FrancisSolvang 10 місяців тому +1

    Good teacher

  • @nagendrakondeti1631
    @nagendrakondeti1631 11 місяців тому +1

    Students in the modern era do they have such a patience writing all steps. I have not seen it in USA. All they use scientific calculators. I told my kids several times to write all the steps. They always say they don't need it. They don't use properly the equal signs, too

  • @nav2014
    @nav2014 8 місяців тому

    Успел заснуть. Нет фантазии у человека и язык безбожно исковеркан.

  • @colt4667
    @colt4667 Рік тому +1

    Very nice solution. Nice handwriting too.

  • @vm_3
    @vm_3 9 місяців тому

    Wow! So complicated solution for so easy task...
    When you know that (x + 6)(x + 5)(x + 4)(x + 3) = 1680 you can just manually verify integer values [2...6). And 1st is our solution.

  • @damiengreenwood7378
    @damiengreenwood7378 8 місяців тому

    Simplest way.,.
    6-2 =4..
    4 factorial = 24..
    (x+6)!=24x1680... Or 40320.. 8! =40320 . Therefore x=2...
    No algebra..just basic arithmetic...about 15 secs....

  • @DavidVonR
    @DavidVonR 11 місяців тому +3

    I solved this one mentally 😅

  • @siyeducation
    @siyeducation 10 місяців тому +2

    Two methods:
    Solving Algebraically (long but thorough, shown in video - beautifully) and solving arithmetically (works quickly for 1680).

    • @rainerzufall42
      @rainerzufall42 10 місяців тому +1

      Don't forget number theory! As it's x! and not gamma function, we have natural numbers in play!
      The prime factors of 1680 show: 1680 = 2^4 * 3 * 5 * 7 and have to be the product of 4 consecutive natural numbers.
      Easy to see, that this has to be 5 * 6 * 7 * 8 = 40 * 42 = 1680.
      You could also substitute u = x + 4.5 (the mean value) and write
      1680 = (u - 1.5) (u - 0.5) (u + 0.5) (u + 1.5) = (u - 1.5) (u + 1.5) (u - 0.5) (u + 0.5) = (u² - 1.5²) (u² - 0.5²)
      and with a formula for (x - a²) (x - b²) = (x - (a²+b²)/2)² - ((a²-b²)/2)² = (x - d² - m²)² - (2dm)² with m = (a+b)/2 and d = (a-b)/2...
      1680 = (u² - 1.5²) (u² - 0.5²) = (u² - 0.5² - 1²)² - 1² = 1681 - 1² = 41² - 1² = (41 + 1) (41 - 1) = 42 * 40 = 6 * 7 * 8 * 5. Ergo:
      u² = 41 + 0.25 + 1 = 42.25
      u = 6.5
      x = 6.5 - 4.5 = 2
      For the calculation:
      a = 1.5
      b = 0.5
      d = (1.5 - 0.5) / 2 = 0.5
      m = (1.5 + 0.5) / 2 = 1
      2dm = 2 * 0.5 * 1 = 1

    • @rainerzufall42
      @rainerzufall42 10 місяців тому +1

      Actually it's even easier with the similar formula for (x - A) (x - B) = (x - M)² - D² with M = (A+B)/2 and D = (A-B)/2. So easy, that I've overlooked that!
      A = 1.5² = 2.25
      B = 0.5² = 0.25
      M = 2.5 / 2 = 1.25
      D = 2.0 / 2 = 1.00
      1680 = (u² - 1.5²) (u² - 0.5²) = (u² - 1.25)² - 1² = 41² - 1 => u = sqrt(42.25) = 6.5 = x + 4.5 => x = 2.

  • @herminiogarcia3788
    @herminiogarcia3788 11 місяців тому +1

    Al llegar a la ecuación de x2+9x-22=0
    es muy simple de factorizar y obtener los valores que se buscan; porque los factores buscados son 11 y -2, ya que al multiplicarlos y restarlos, pues es uno positivo y otro negativo, nos da los elementos del trinomio que tenemos...
    11 x (-2) = -22 y 11 - 2 = 9
    Por lo que no le veo el caso a lo que hizo después de esa ecuacion cuadrática.

  • @aines7958
    @aines7958 11 місяців тому

    🎉🎉🎉🎉👍🌹🇩🇿

  • @Sg190th
    @Sg190th 10 місяців тому

    This is well written.

  • @MrEnniscorthy
    @MrEnniscorthy 10 місяців тому

    great

  • @manishsaxena4566
    @manishsaxena4566 Рік тому +1

    Solution video Bahut achcha hai ; Lekin 0:55 min par video me ((x+2)!) likh te to Perfect solution hota

  • @alainbrochet3120
    @alainbrochet3120 11 місяців тому +2

    1680 est le produit de quatre entiers successifs et la racine cubique de 1680 est sensiblement 6,4 qui est compris entre 6 et 7, donc il est facile de déduire que les quatre entiers sont 5,6,7 et 8 (trente secondes qui dit mieux ?).

  • @charlesmitchell5841
    @charlesmitchell5841 Рік тому

    Nice problem. Thanks for the lesson. 👍

  • @ouedraogosaiyouba5709
    @ouedraogosaiyouba5709 Рік тому

    2

  • @bogusawsroda3747
    @bogusawsroda3747 Рік тому

    Super

  • @cachotrelles4715
    @cachotrelles4715 11 місяців тому

    👏👏👏👏

  • @Frisbieinstein
    @Frisbieinstein 10 місяців тому

    Approximate the fourth root. It's about 6. 1680 is divisible by 5, so that's a factor. Guess 5 6 7 8. Remember to subtract 2.

  • @jayeshkumar3861
    @jayeshkumar3861 10 місяців тому

    We need 4 consecutive numbers that are multiplied to get 1680. Means the numbers are close to the 4th root of 1680 ie square root of 40+ ie 6+ that's simply 6, 7, 5 (as we need 0) and remaining u can see 8... ... Thus, x is 2....
    8!/4! ie (2+6)!/(2+2)!

  • @igorrodrigues6253
    @igorrodrigues6253 11 місяців тому

    Congrats!

  • @syedmdabid7191
    @syedmdabid7191 Рік тому +1

    ( x+3) ( x+4) ( x+5) ( x+6) = 16 80 and then go to Uncle FERRARI..

  • @miriamvianaesilva1118
    @miriamvianaesilva1118 11 місяців тому

    Passa tempo aprovado.

  • @mathwithmelissarose
    @mathwithmelissarose 10 місяців тому

    Thanks for making this video!

  • @cranberry51
    @cranberry51 Рік тому +2

    Very insightful!
    The crucial step is probably the successful attempt to make the expressions in brackets equal and then to substitute them. The formula used to determine the (real) solution to the quadratic equations
    I replaced it with the formula: minus p/half plus/minus root of p/half squared minus q, I can deal with that better.
    Thanks for the instructive lecture.

  • @gecumenico
    @gecumenico Рік тому

    Excelente... 🇻🇪

  • @goldfing5898
    @goldfing5898 Рік тому +2

    13:50 This test can be done much easier. We have
    8 * 7 * 6 * 5 = 1680
    Rather than tediously multiplying 56 * 30, just divide by 8:
    7 * 6 * 5 = 1600/8 = (1600 + 80)/8 = 200 + 10 = 210
    Then write
    7 * 30 = 210
    And you are done.

  • @TGM_12345
    @TGM_12345 Рік тому +3

    Nice ... Why the quadratic formula of the equation x² + 9x -22 does not yield the solutions ?

    • @samuelbenet007
      @samuelbenet007 Рік тому +1

      Parce Δ

    • @88kgs
      @88kgs Рік тому

      It has complex solution/s, since the discriminant is less than zero, and the examiner in the starting of the video is asking for the real solution/s... For that reason it is rejected 👍

    • @ffggddss
      @ffggddss 10 місяців тому +1

      Quadratic formula *does* yield solutions for that quadratic.
      ∆ = b² - 4ac = 81 + 88 = 169 = 13²
      x = ½(-b ± √∆) = ½(-9 ± 13) = ½{4, -22} = {2, -11}
      x = 2 is valid; x = -11 is not because the factorials in the original equation become factorials of negative integers, which are undefined / ±∞.
      Fred

  • @Martin-hi7wx
    @Martin-hi7wx 11 місяців тому

    Pedagogic .....

  • @nickcampbell3812
    @nickcampbell3812 10 місяців тому

    Let's See.
    The division of the factorial terms leaves the product
    x+6 * x+5 * x+4 * x+3
    Which means I am looking for 4 consecutive numbers that multiply to be this number.
    It is obvious that this number is divisible by 80 since 16 divides 8 and 80 divides 80 which gives 80*21=1680
    Decomposing these 2 factors gives
    2^4 *5 *3 *7
    Now I have 7 multiplications that I need to reduce to 4 consecutive multiplications.
    I'll start with the lowest number 2 and keep multiplying it by 2s until it is no longer lowest, which gives
    2^2 * 3*4*5*7
    Now I multiply the 3, which is the new lowest, by 2
    2^1 * 4*5*6*7
    Now I multiply the new lowest which is 4, by 2 to get
    5*6*7*8 = 1680
    Now the highest term in this product must be x+6 = 8 meaning x=2.
    8! /4! = 8*7*6*5 = 1680
    I find it interesting how many ways there are ro solve this. I wrote my solution before watching the video or checking the comments, and it is astounding how many other ways people came up with.

  • @cachotrelles4715
    @cachotrelles4715 11 місяців тому

    Al final de resultado aclaré y reafirmé concepto. Gracias

  • @voidxvoid
    @voidxvoid 10 місяців тому

    1680^(1/4)=6~7

  • @nasrullahhusnan2289
    @nasrullahhusnan2289 Рік тому

    (x+6)!/(x+2)!=1680
    (x+6)(x+5)(x+4)(x+3)=1680
    Note that LHS is a product of four successive numbers, and one of them must be 5 or 10 as 10 | 1680.
    Note that 1680=16×105
    =2×8×3×5×7
    =5×6×7×8
    The least factor (x+3) is 5 --> x=2
    As there are four factors, negative factor is also possible:
    1680=(-5)×(-6)×(-7)×(-8)
    The greatest factor is (x+6). Thus
    x+6=-5 --> x=-11
    Another method:
    Group the factors into two: the first with the last and the other two in the center. Multiplying the factors in each group we get
    (x²+9x+18)(x²+9x+20)=1680
    [(x²+9x+19)-1][x²+9x+19)+1]=1680
    (x²+9x+19)²-1=1680
    (x²+9x+19)²=1681
    =41²
    x²+9x+19=±41
    x²+9x-22=0 --> (x+11)(x-2)=0
    x=2 or x=-11
    x²+9x+60=0 --> no real valued solution

    • @RoderickEtheria
      @RoderickEtheria 11 місяців тому

      Are there negative factorials?

    • @nasrullahhusnan2289
      @nasrullahhusnan2289 11 місяців тому

      @@RoderickEtheria: I have not vome across negative factorial. But I think it is possible to have (-n)! I hope that you are familiar with combination to follow my argument.
      Combination of n objects taken k each time.
      There is a symbol for that sentence, but I have difficulty of writing the symbol here. Instead I will write it as nCk defined as
      nCk=n!/[k!(n-k)!]
      =n(n-1)(n-2)...(n-k+1)/k!
      There is formula -nCk as
      -nCk=(-n)(-n-1)(-n-2)...(-n-k+1)/k!
      =[(-1)^k]n(n-1)(n-2)...(n+k-1)/k!
      =[(-1)^k](n+k-1)Ck
      You will find this in Negative Binomial Distribution.
      Do you think that
      (-n)!=[(-1)^n]n!
      =n! if n is even
      =-n! if n is odd

    • @RoderickEtheria
      @RoderickEtheria 11 місяців тому

      @nasrullahhusnan2289 I don't think negative factorial would work here either even if negative factorial was possible. Negative factorial, if possible, would have to head towards -1. But, in this case, the denominator would be farther from -1 than the numerator and should create a fraction less than 1.

    • @nasrullahhusnan2289
      @nasrullahhusnan2289 11 місяців тому

      @@RoderickEtheria: You are right if x=-11 is plugged back to the original equation: (x+6)!/(x+2)!=1680. But it fits to the simplified equation
      (x+6)(x+5)(x+4)(x+3)=1680
      as LHS=(-5)(-6)(-7)-(8)
      =5×6×7×8 as there are 4 minus
      =1680
      It's surprise me.

    • @muontran8589
      @muontran8589 10 місяців тому +1

      Đoạn chót dài dòng quá,nên dùng công thức giải phương trình bậc hai sẽ nhanh hơn!

  • @adgf1x
    @adgf1x 8 місяців тому

    since x is non zero whole no.x=2 and not equals (-11)..

  • @neilmccoy9390
    @neilmccoy9390 Рік тому

    TRIAL BY ERROR. 4th roots of each side are roughly x+4 and either 6 or 7. Try x+4=6. Then LHS=5X6X7X8=1680

  • @daddykhalil909
    @daddykhalil909 10 місяців тому

    9:36 I liked the solution as much as I enjoyed reading all comments.
    Thank you all

  • @cachotrelles4715
    @cachotrelles4715 11 місяців тому

    Muchas veces, el idioma universal es, son, los números y la aplicación de sus fórmulas.👍👍

  • @Psykolord1989
    @Psykolord1989 11 місяців тому

    Before watching:
    Alright, so, (x+6)! = (x+6)(x+5)(x+4)(x+3) (x+2)!. Thus, we can rewrite the left hand side as:
    (x+6)(x+5)(x+4)(x+3) = 1680
    Now, we have some options. We know that (x+6)(x+5)(x+4)(x+3) are a set of 4 consecutive numbers. I think that while we could go through and find the factors of the quartic equation we'll wind up with, it may behoove us to simply factorize 1680.
    1680 = (10)(168)
    = (2)(5)(84)(2)
    = (2)(5)(3)(7)(2)(2)(2)
    Now, 5 and 7 are only 2 apart. If we can rearrange the remaining 2s and the 3 to make either 6 and 8 or 4 and 6, we're good. Fortunately, we can!
    1680 = (5)(7)(2)(2)(2)(2)(3) = (5)(7)(2*3)(2*2*2) = (5)(7)(6)(8) = (5)(6)(7)(8) = (x+3)(x+4)(x+5)(x+6)
    Thus, X=2 is our answer.

  • @miriamvianaesilva1118
    @miriamvianaesilva1118 11 місяців тому

    PS😊s😅a g

  • @PeteMachini6732
    @PeteMachini6732 10 місяців тому

    7:20, shouldn't you bring the 19 over the equal sign instead of the 41?

  • @cachotrelles4715
    @cachotrelles4715 11 місяців тому

    Buen día. PREGUNTO: LA RAIZ CUADRADA DE UN NÚMERO ENTERO POSITIVO, NO DA COMO RESULTADO OTRO NÚMERO POSITIVO???

    • @herminiogarcia3788
      @herminiogarcia3788 11 місяців тому

      No necesariamente, generalmente son los positivos los que usamos, pero también pudieran ser negativos, que casi nunca usamos, porque dos números positivos, lógico que al multiplicarlos da positivo; pero dos números negativos, al multiplicarlos, también dan un número positivo.

    • @nahuelastor7522
      @nahuelastor7522 11 місяців тому

      Depende si se lo preguntas a un profesor de Algebra o a uno de Análisis 😂

  • @michaelpenklis3104
    @michaelpenklis3104 10 місяців тому

    Just remember what the Factorials are 1,2,6,24,120,720.....

  • @Metal_dead
    @Metal_dead 10 місяців тому

    It's better to write n instead of x as n is natural number

    • @Metal_dead
      @Metal_dead 10 місяців тому

      Sorry not natural, but still integer

  • @johnfederkins4177
    @johnfederkins4177 10 місяців тому

    Save yourself 14 minutes and some ink & paper. As others said, just start with an estimate from the 4th root.

  • @gothargupta2.0
    @gothargupta2.0 11 місяців тому

    6714+6 : 2+2 =1680

  • @mahmodragib
    @mahmodragib 11 місяців тому

    x+6p4=8p4 x+6=8 x=2

  • @okko104
    @okko104 Рік тому

    I have aout 2xes

  • @Khaledasad1963
    @Khaledasad1963 11 місяців тому

    There are much shorter way imagine getting this on test toooooo long

  • @kellyrosy8256
    @kellyrosy8256 Рік тому

    Tuyệt

  • @吳昶霖
    @吳昶霖 Рік тому

    X=2

  • @miracbarsapaydn650
    @miracbarsapaydn650 Рік тому

    X = 5

    • @swapansanyal4951
      @swapansanyal4951 Рік тому

      x=5 does not satisfy the given equation.So x=5 is not the solution

    • @lornawilliams550
      @lornawilliams550 Рік тому

      ​@@swapansanyal4951
      If x=5 then answer would be 7920 which is incorrect

  • @JohnStrandt
    @JohnStrandt 10 місяців тому

    Slow down chief, ya lost me at XC square.

  • @v.krishnamurthib610
    @v.krishnamurthib610 11 місяців тому

    I am not able to understand this sum

  • @numerikos_silva
    @numerikos_silva Рік тому

    I have two ways more to resolve it in my channel NUMERIKOS

  • @spoilercollectersyllogi.ki6806
    @spoilercollectersyllogi.ki6806 8 місяців тому

    Are you mathematician ?

  • @benjohnson4755
    @benjohnson4755 10 місяців тому

    Lol 😂😂😂😂splitting middle term is joking or what 😂😂😂😂

  • @comando699
    @comando699 11 місяців тому

    Why.x=-b+-√∆÷2a

  • @asmaaftab4497
    @asmaaftab4497 11 місяців тому

    X=1.996

  • @archilk.khoperia81
    @archilk.khoperia81 9 місяців тому

    Глупо! Очень глупо! Я без формул за 20 секунд решил. Почему глупо? - а в друг будет выражение х-13 и х-4 тогда что, какие будут формулы?

  • @dileepmv7438
    @dileepmv7438 11 місяців тому

    Why pronouncing x^2 as “exci square” , but x as “ex”

  • @jeffreyschmiedeck4254
    @jeffreyschmiedeck4254 11 місяців тому

    You should get someone to explain this in English!

  • @hmongculture9042
    @hmongculture9042 Рік тому

    This problem is not a condition of X>= -2, because when X>= -2 the denominato is zero and is not allowed! It must be X >-2! !!👎👎👎

    • @prashantgupta43905
      @prashantgupta43905 11 місяців тому

      Notice it's (x+2)!
      Even if x+2=0, we get 0!=1
      So (x+2)>=0 is not wrong 👍🏻

  • @АндрейКолычев-ш8м
    @АндрейКолычев-ш8м 9 місяців тому

    Super

  • @soonhyeokhong6294
    @soonhyeokhong6294 Рік тому

    X=2

  • @Bertin-q3y
    @Bertin-q3y 11 місяців тому

    X=2

  • @mrmimi807
    @mrmimi807 11 місяців тому

    X = 2