Consider the proof. given: in △abc, bd ⊥ ac prove: the formula for the law of cosines, a2 = b2 + c2 – 2bccos(a) statement reason 1. in △abc, bd ⊥ ac 1. given 2. in △adb, c2 = x2 + h2 2. pythagorean thm. 3. in △bdc, a2 = (b – x)2 + h2 3. pythagorean thm. 4. a2 = b2 – 2bx + x2 + h2 4. prop. of multiplication 5. a2 = b2 – 2bx + c2 5. substitution 6. in △adb, cos(a) = 6. def. cosine 7. ccos(a) = x 7. mult. prop. of equality 8. a2 = b2 – 2bccos(a) + c2 8. ? 9. a2 = b2 + c2 – 2bccos(a) 9. commutative property what is the missing reason in step 8?