The picture below is an image of z = f(x, y). Its partial derivatives are fr = -2r and fy = -2y. (a) Write the equation for the tangent plane at the point (.5, .5, 1.5) and sketch it on the graph. X A (b) Use a differential or tangent plane approximation to estimate the value of f(1, 1). Then draw the point (1, 1, *) on your graph (for * your approximated value). (c) Is your approximation from part (b) good? Why/why not