Heron's Formula |

This Math in Minutes video demonstrates how to calculate the area of a triangle when no height is given, only the lengths of the 3 sides are provided.

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Another formula is:
Let p = (ab)^2 + (ac)^2 + (bc)^2 and q = a^4 + b^4 + c^4
Area A = 1/4 sqr(2p - q)