*Angle θ is in quadrant II and sin θ **= **\({5 \over 13}\)** .** Determine an exact value for cos 2θ.*

Since angle θ is in quadrant II for cos 2θ, I used the CAST rule and I inquired that this value must be negative.

I also used the *double angle identity *of **cos 2A = ****cos****\({^2}\)****A**** - ****sin****\({^2}\)****A**.

If sin θ = *\({5 \over 13}\)*, I subbed it in for A and multiplied it by 2.

I got **- cos (\({10 \over 26}\))**, but the answer in the textbook is **\({119 \over 169}\) . **How should I solve this?

Guest Feb 22, 2019