Question

Question #9f971

Answer 1

It is `-sqrt3/2`

Explanation:

It is

#cos (7pi/4)cos(7pi/12)+sin(7pi/4)sin(7pi/12)=cos(7pi/4-7pi/12)= cos(7*pi/6)=cos(pi+pi/6)=-cos(pi/6)=-sqrt3/2#

Note

We used the identity

`cos(A-B)=cosA*cosB+sinA*sinB`