Use the base–height formula: (A=tfrac{1}{2}bh)
Identify the base (b) and the perpendicular height (h) (distance from the base to the opposite vertex)
If you have side lengths (a,b,c), use Heron’s formula:
(s=tfrac{a+b+c}{2})
(A=sqrt{s(s-a)(s-b)(s-c)})
If you have two sides and the included angle (theta), use: (A=tfrac{1}{2}absin(theta))
If the triangle is on a coordinate plane with vertices ((x_1,y_1),(x_2,y_2),(x_3,y_3)), use:
(A=tfrac{1}{2}left|x_1(y_2-y_3)+x_2(y_3-y_1)+x_3(y_1-y_2)right|)
If the triangle is right with legs (a) and (b), use: (A=tfrac{1}{2}ab)