16.7 #4 Find the absolute value of the Jacobian for

	x = s^3 - 3st^2
	y = 3s^2t - t^3

answer

partial(x,y)   | 3s^2-3t^2       -6st |
------------ = |                      |
partial(s,t)   |    6st      3s^2-3t^2|


= (3s^2-3t^2)^2 + 36s^2t^2 = (3s^2 + 3t^2)^2 = 9(s^2 + t^2)^2