a) sin2(x)=½, sin(x)=±1/√2, x=π/4, 3π/4, 5π/4, 7π/4 (45°, 135°, 225°, 315°)
b) cos2(x)=-cos(x), cos(x)(cos(x)+1)=0, cos(x)=0, x=π/2, 3π/2 (90°, 270°); cos(x)=-1, x=π (180°).
c) 6sin2(x)-11sin(x)+4=0=(2sin(x)-1)(3sin(x)-4), sin(x)=½, x=π/6, 5π/6 (30°, 150°).
d) 3cos2(x)-2cos(x)-1=0=(cos(x)-1)(3cos(x)+1), cos(x)=1, x=0, 2π (0°, 360°); cos(x)=-⅓, x=1.91 (109°), 4.37 (251°).