Proof

Can you tell me how to proof the following fomulae:
1. volume of pyramid ------ 1/3 × A ×h
A = base area , h = height
2.volume of cone ---------- 1/3 ×π × r^2 × h
r = radius h = height
3.volume of sphere -------- 3/4 × π × r^3
r = radius
Thank you.