Example :9 "c 2 2r-1 if a,=|m*-1 71 1+| then 4, is equal to r=0 sin (n) sin (m) sin (um +1) (a) m- 1 (c) zero (b) 2 (d) none of these please explain .

Example :9 "C 2 2r-1 If A,=|m*-1 71 1+| then 4, is equal to r=0 sin (n) sin (m) sin (um +1) (a) m- 1 (c) zero (b) 2 (d) none of these