Question

Diketahui tan a° = 3/4 dan 180 < a < 270, maka nilai cos 1/2a° sama dengan?

Answer 1

Tan a = 3/4 = y/x  dan a di KD III
tripel pitagoras  x,y, r = 4,3,5

cos a = - x/r = - 4/5

cos a = 2 cos² 1/2 a  - 1 
cos 1/2 a = √ { 1/2 (cos a + 1)}

cos 1/2 a = √ { 1/2 ( - 4/5 +1)
cos 1/2 a = √ {1/2(1/5) = √(1/10)
cos 1/2 a = 1/10 √10
krn a dikuadran III---> maka cos 1/2 a = - 1/10 √10