Answer:
cos(u+v)=cosucosv-sinusinv
here sinu=12/13
therefore cosu=12/13(+ve because 1st quadrant)
by using triangle and Pythagoras theorem
and cosv=-3/5(-ve 2nd quadrant cos -ve)
sinv=4/5(+ve 2nd quadrant)
now according to formula
cos(u+v)=(5/13*-3/5)-(12/13*4/5)
=-63/65
