Review: Heron's formula

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Learn it Heron's formula finds a triangle's area from its three sides: $s = \frac{a + b + c}{2}$, then $A = \sqrt{s(s - a)(s - b)(s - c)}$. No height needed!

Example: Find the area of a triangle with sides 13, 14 and 15 cm.

$s = (13 + 14 + 15) \div 2 = 21$.
$A = \sqrt{21 \times 8 \times 7 \times 6} = \sqrt{7{,}056}$.
$A = 84$ cm².

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