Question 1094865
COS (X+Y)= COSXCOSY-SINXSINY;
 IF (a,b) be a position vector on xy plane .
 sin(angle) = b/sqrroot( a^2 + b^2);
 cos (angle)= a/sqrroot(a^2 +b^2);
according to question cosx = 3/5  (base/ hyp);
                       sinx = -4/5 (iv quad) ;
similarly siny= -2/sqrroot5;(per/hyp)
          cosy=  -1/sqrroot5;(base/hyp);
 now cos(x+y)= (3/5)(-1/sqrroot5)-(-4/5)(-2/sqrroot5);
             -11/5sqrroot5;