4. Find an equation for the tangent plane to the graph of $f$ at the point
$P = (x_0, y_0, f(x_0, y_0))$.
(a) $f(x, y) = \ln(x^2 + y^2)$, $P = (1, 0, 0)$.
(b) $f(x, y) = e^{-(x^2 + y^2)}$, $P = (0, 0, 1)$.
(c) $f(x, y) = \sqrt{y - x}$, $P = (1, 2, 1)$.
(d) $f(x, y) = \cos(x + y)\sin(x - y)$, $P = (0, 0, 0)$.
(e) $f(x, y) = \frac{2x + y + 1}{x^2 + y^3}$, $P = (1, 1, 2)$.
