Question

Find cos(2x) from the given information. tan(x)= 9/8, x in quadrant I

Answer 1

cos2x=-17/145

Explanation:

Recall cos2x=cos^2x-sin^2x

Or cos2x=cos^2x-(1-cos^2x)*

Or cos2x=2cos^2x-1**

*By a Pythagorean Identity

**Combined like terms

I'm going to use third identity from above because I only have to find cosx or cos^2x to get requested answer for cos2x.

Recall Pythagorean identity 1+tan^2x=sec^2x.

Plug in our tangent valuem...

1+(9/8)^2=sec^2x

1+81/64=sec^2x

145/64=sec^2x

Cosine and secant are reciprocals of each other.

64/145=cos^2x

Now we are ready to plug in and get final answer:

cos2x=2cos^2x-1

cos2x=2(64/145)-1

cos2x=128/145-1

cos2x=-17/145