Question

Найдите 49cos2a, если cosа=2/7

Answer 1

$49cos2a=49(cos^{2}a-sin^{2}a)\\\\cosa= \frac{2}{7}\\sin^{2}a=1-cos^{2}a\\sin^{2}a=1- (\frac{2}{7})^{2} \\sin^{2}a= \frac{45}{49} \\\\49(cos^{2}a-sin^{2}a)=49( \frac{4}{49}- \frac{45}{49})\\49* (-\frac{41}{49})=-41$