Đáp án:
Giải thích các bước giải:
cos (6x) . cos (2x) = sin (7x) . sin (3x)
⇔ $\frac{1}{2}$ (cos (8x) + cos (4x)) = $\frac{1}{2}$ (cos (4x) - cos (10x))
⇔ $\frac{1}{2}$ (cos (8x) + cos (4x)) - $\frac{1}{2}$ (cos (4x) - cos (10x)) = 0
⇔ $\frac{1}{2}$ cos (8x) + $\frac{1}{2}$ cos (10x) = 0
⇔ $\frac{1}{2}$ (cos (8x) + cos (10x)) = 0
⇔ cos (8x) + cos (10x) = 0
⇔ cos (8x) = -cos (10x)
⇔ cos (8x) = cos ($\pi$ - 10x)
⇔ 8x = $\pi$ - 10x + k2$\pi$
⇔ x = $\frac{\pi}{18}$ + $\frac{k\pi}{9}$