As well as comparing the shapes of the formulae you can try interpreting the equation visually. You can see the apex is at (0,0,2) (can you? I mean, you'll minimize the subtracted sqrt expression only by setting x and y to zero). Also, you can see that z is zero where x^2 + y^2 = 2^2... Also, over the x-axis, where y = 0, you have a straight line: z = 2 - x...