3. Recall that a tangent plane is the linear approximation to a differentiable function. (a) [3 points] Suppose that f(x, y) is twice continuously differentiable. Using your un- derstanding of linear approximations as a guide, give what you think is a reasonable definition for the quadratic approximation to f(x, y) at the point (a, b). (b) [3 points] Give a formula for the quadratic approximation to f(x, y) at the point (a, b). Hint: your formula should contain three quadratic terms, in addition to the constant and linear terms from the tangent plane