We already know ∠1, but would it be possible to solve the problem if we didn't? Sure thang! Take a look at ∆*ABD*. One angle is 21°, another is ∠3, and the top angle is 85° + 17°. (We can do that because of the Angle Addition Postulate.) So, in case we've forgotten that ∠1 is 38°, we can still solve this using the Angle Sum Theorem. m∠3 + m∠*BDA* + m∠*ABD* = 180 m∠3 + 21 + (85 + 17) = 180 m∠3 = 57 Wasn't that a (triangular) piece of cake? |