Membrane elasticity (20 points). 2. Consider a membrane described by the height function h(x1, X2) = -3x; + 5x3. Compute the principal curvatures for this membrane, and compute the bending free energy (as a function of Kb) for a piece of membrane that spans the interval x1 from -5 to 5 and x2 from 0 to 5. (10 points)