Question

Find the exact value of each expression:
cos[tan^-1 (-1) + cos^-1 (-4/5)]

I know that tan inverse of -1 is -pi/4
Will FAN and MEDAL for help

Answer 1

Use the sum and difference identity for cosine
\[\cos(u+v)=\cos b \cos v-\sin u \sin v\]

and use this triangle to figure out sin (cos^-1 (-4/5))