Question

If in oblique ΔABC
side a=13
m, side b=6
m, and side c=9
m, then use the Law of Cosines to find the measure of ∠A
.(1 point)

119 degrees

50 degrees

63 degrees

15 degrees

Answer 1

Use the Law of Cosines:
cos A = (b^2 + c^2 − a^2) / (2bc) = (6^2 + 9^2 − 13^2) / (2·6·9)
= (36 + 81 − 169) / 108 = −52/108 = −13/27.

So A = arccos(−13/27) ≈ 119°.