А) 4 - 3sin²(4x - 5) = 0
4 = 3sin²(4x - 5)
sin²(4x - 5) = 4/3
Нет корней, т.к. sina ∈ [-1; 1].
б) 2sin²2x + 7cos²2x = 3
2sin²2x + 2cos²2x + 5cos²2x= 3
2 + 5cos²2x = 3
5cos²2x = 1
cos²2x = 1/5
cos2x = ±√5/5
2x = ±(arccos±√5/5) + 2πn, n ∈ Z
x = ±1/2(arccos±√5/5) + πn, n ∈ Z
(или x = 1/2(arccos(-√5/5)) + πn и 1/2(arccos √5/5) + πn, n ∈ Z).