5sin^2x-11cisx^2+3=0

5sin²x-11cos²x+3 = 0

5sin²x-11 (1-sin²x) + 3 = 0

5sin²x-11+11sin²x+3 = 0

16sin²x-8 = 0

16sin²x = 8

sin²x = 1/2

sin x = 1/√2 и sin x = - 1/√2

sin x = 1/√2 → x = (-1) ^n·π/4+πn

sin x = - 1/√2 → x = (-1) ^n·5π/4+πn, n∈Z