Step-by-step explanation:
7-3cosx=9sin²x
[sin²x=1-cos²x]
=>
7-3cosx=9-9cos²x
9cos²x-3cosx-2=0
let cosx=t
9t²-3t-2
by quadratic formula,
t=(3±9)/2·9
=> t=2/3 or t=-1/3
thus,
x=acos(2/3) or x=π- acos(1/3)
