Area of triangles

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Learn it $A = \frac{1}{2}bh$, with h perpendicular to the base. On a grid, use $A = \frac{1}{2}|x_1(y_2 - y_3) + x_2(y_3 - y_1) + x_3(y_1 - y_2)|$ or a box around the triangle.

Example: Find the area of the triangle A(1, 1), B(6, 2), C(3, 5).

$\frac{1}{2}|1(2 - 5) + 6(5 - 1) + 3(1 - 2)|$
$= \frac{1}{2}|-3 + 24 - 3| = \frac{1}{2} \times 18$
$= 9$ square units.

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