Question

Найдите 25cos2α, если sinα=−0,7

Answer 1

sina=0.7
cos2a=cos^2 a-sin^2
a cosa=sqrt(1-sin^2 a)=sqrt(1-0.49)=sqrt(0.51).
cos2a=0.51-0.49=0.02.
25cos2a=25*0.02=0.5

Answer 2

$25cos ^{2} \alpha =25*(cos ^{2} \alpha -sin ^{2} \alpha )=25*(1-sin ^{2} \alpha -sin ^{2} \alpha )=25*$$*(1-2sin ^{2} \alpha )=25-50sin ^{2} \alpha$

Подставляем,
$25-50*(-0,7) ^{2}=25-50*049=25-24,5=0,5$