Question 26 in SAT Digital Practice Test #3, MATH Section 2 Module 2

This video explains the answer to Question 26 from the SAT Digital Practice Test 3 Section 2 Module 2 Math portion. The question is −9x^2 + 30x + c = 0 In the given equation, c is a constant. The equation has exactly one solution. What is the value of c ?This problem involves quadratic equations, discriminant of the quadratic formula, and solving for x. Calculators ARE allowed on this section. This practice test is provided for free by the College Board. This video will show you how to do this problem and the logic behind it.
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[@V_CTR]

Thank you so much for this series I've been keeping track since question 1 and it has helped me prepare for my SATS coming up and I just wanted to say thank you! If it wasn't for you I'd be lost!

[@YouCanLearnMathwithAlyssa]

That’s so kind of you to say - I’m so glad they’ve helped! 😊

[@MrSummitville]

No, the square root of 9 is 3, not +3 and -3. It is the +/- in front of the square root that provides the two solutions.

[@YouCanLearnMathwithAlyssa]

That’s what so much fun about math- there is often more than one way to express an answer 😊. Like how the square root of x and x to the power of 1/2 are the same. In this problem, +- 3 or 3 and negative 3 are the same, so we’re free to use which one we prefer. While I chose to use the two separate numbers in this problem, I’m sure you’ll find other problems on my channel where I used +- 😉

[@MrSummitville]

You claim the square root of a negative number is "impossible". Yet, electrical engineers use the sqrt(-1) or i ... everyday.

[@YouCanLearnMathwithAlyssa]

In this problem, we are told the equation has one solution. With quadratics, this means one *real* solution. Imaginary solutions using i would not apply here, so a square root of a negative number would be an impossible answer for this particular problem. In other situations, we would absolutely consider any imaginary solutions 😊