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?
