Starting from the formula r.. = -μ(r/γ3) show by differentiation that the quantity h =|r.|2/2- μ/γ is a constant. It is useful to recall that |r.|2= r..r.. You will need to use your calculus skills to find an expression for the derivative of 1/r with respect to time t, after which the fact that γγ.= r.r. is likely to be useful.