Question

Using the law of Cosines, in triangle DEF, if e=18 yd, d=10 yd, f=22 yd, find angle D

Answer 1

`d^2=e^2+f^2-2ef \cos D`


`d^2-e^2-f^2=-2ef \cos D`


`\cos D = (e^2+f^2-d^2)/(2ef)`


`\cos D = (18^2+22^2-10^2)/(2(18)(22)) = 59/66`


`D= \arccos (59)/(66) \approx 26.63^\circ`