Question

Зная, что cosx=8/13 и x∈(3π/2;2π), вычисли: cos2x−6,9

Answer 1

$cosx=\frac{8}{13}\; \; ,\; \; x\in (\frac{3\pi }{2},2\pi )\\\\cos2x=2cos^2x-1=2\cdot \frac{64}{169}-1=-\frac{41}{169}\\\\cos2x-6,9=-\frac{41}{169}-\frac{69}{10}=-\frac{12071}{1690}=-7\frac{241}{1690}$