Let a and b be two constant vectors. Show that(the integration is over the usual range: O&...

Let a and b be two constant vectors. Show that

(the integration is over the usual range: O<0<7r,O<0<27r). Use this result to demonstrate that

for states with l = 0. Hint: r = sin θ cosϕ i + sin θ sin ϕ j + cos θk.