Convolution

[tronxo]

I've done few examples about convolution and i understand it grafically but not mathematically. when two rectangular functions are convolved, why is the result a triangular function?? given this example:
x(t)= A*"rectangular function of" (t/T)
h(t)= B*"rectangular function of" (t/B)
could anyone prove that if those functions are convolved the result is a triangular function? i ve been working on it for a while and i havent been able to give a valid result