Question

已知cos@=-五分之四 且@属于π 二分之三π 球cos二分之@ 咳咳 怎么弄啊

Answer 1

cosa=-4/5

a∈(2π/3,π)

a/2∈(π/3,π/2)

cosa/2>0

cosa=2(cosa/2)^2-1

-4/5=2(cosa/2)^2-1

2(cosa/2)^2=1/5

(cosa/2)^2=1/10

cosa/2=√10/10