8(sin²x)² +13sin²x -7 =0 ;
t =sin²x ; 0 0≤ t ≤ 1 ;
8t² +13t -7 =0 ;
D =13² - 4*8(-7) =393.
t = (√393 -13)/16 * * * * * (-13 - √393)/16 <0 * * * * *<br>sin²x =(√393 -13)/16 ;
(1 - cos2x)/2 =(√133 -13)/16 ;
cos2x =(21 -√393)/8 ;
2x =±arccos(21 -√393)/8 +2π*k , k ∈ Z ;
x = ±1/2*arccos(21 -√393)/8 + π*k, k ∈ Z .