Right so if we differentiate our demand function  we get that alright, nothing that for our elasticity we want one over D P D Q  okay, so our elasticity you could write as one over minus beta A Q to the minus beta plus one  right, times our price quantity ratio which if we now just substitute in the price, so that we have got A Q to the minus beta, right over Q , right, that equals  A Q to the minus beta minus one over minus beta A Q minus beta to the minus one right which cancels to give us minus one over beta.