Homework #9

13.7    Chain rules
	10. Compute partial derivative of a composite function.
	    Just do the derivative w.r.t. y.
        18. Write chain rules.
        24. Implicit partial differentiation.
        38. Resistors.
13.8    36. Gradient of a product.
        44. Directional derivative.
        46. Plane tangent to a paraboloid and parallel to another plane.

OMIT!! 13.9    36.  Maximization of triangle area with fixed perimeter.
        Hint: A2 will be easier to maximize than A itself.
	You have to a rather tricky set of nonlinear equations. I started by
	eliminating the multiplier lambda between two of the equations.
        Some of the solutions will correspond to totally squashed triangles:
        minima of A2 rather than maxima: disregard these.


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