Will mark brainliest, given: cd is an altitude of △ abc . prove: a^2=b^2+c^2−2bc cos a? (choices are on the bottom) cd is an altitude of △ abc > given △acd and △bcd are right triangles. > a^2=(c−x)^2+h^2 > a^2=c^2−2cx+x^2+h^2 > square the binomial. b^2=x^2+h^2 > pythagorean theorem cosa=x/b > b cos a = x > multiplication property of equality a^2=c^2−2c (b cos a) +b^2 > a^2=b^2+c^2−2bc cos a > commutative properties of addition and multiplication choices: definition of cosine definition of right triangle pythagorean theorem substitution property of equality