Use the Pythagorean theorem for a right triangle: (c = sqrt{a^2 + b^2})
Use the Law of Cosines for any triangle: (c = sqrt{a^2 + b^2 – 2abcos(C)})
Use the Law of Sines when one side and its opposite angle are known: (frac{a}{sin A} = frac{b}{sin B} = frac{c}{sin C})
Use the perimeter formula to find total triangle length: (P = a + b + c)
Use Heron’s formula to find area if all three sides are known: (A = sqrt{s(s-a)(s-b)(s-c)}), where (s = frac{a+b+c}{2})
Use coordinate geometry if vertices are given: (d = sqrt{(x_2-x_1)^2 + (y_2-y_1)^2})
Use the side-angle-side, angle-side-angle, or side-side-side information to determine missing side lengths
Use scale drawings or similar triangles if the triangle is proportional to another known triangle