If cos x=−6/7, and 90°

0.4693877

Step-by-step explanation:

Given that:

cos x=−6/7,

From trigonometry :

Cosx + sinx = 1

Cos2x = cos(x + x)

Recall the double angle identity:

Cos2x = 2cos^2x - 1

Sin2x = 1 - 2sin^2x

Since cosx = - 6/7

Then

Cos2x = 2cos^2x - 1

Cos2x = 2cosx^2 - 1

Cos2x = 2(-6/7)^2 - 1

Cos2x = 2(36/49) - 1

Cos2x = 2 * (72/98) - 1

Cos2x = (2 * 0.7346938) - 1

Cos 2x = ( 1.4693877 - 1)

Cos2x = 0.4693877

step-by-step explanation:

no, her answer is not correct.   we square the lengths of all sides and then determine whether the pythagorean theorem holds here:

9² + 15² = 306.   this is not equal to the square of the longest side ( that square being 395.   answer c is correct.