Supplementary Exercises for Sections 13.6
1. How many third-order derivatives does a function of 2 variables have? How many of these
are distinct?
2. How many nth order derivatives does a function of 2 variables have? How many of these are
distinct?
3. Let α and k be constants. Prove that the function
u(x, t) = e−α
2 k2 t
sin(kx)
is a solution to the heat equation ut = α2 uxx
4. Let a be a constant. Prove that
u = sin(x − at) + ln(x + at)
is a solution to the wave equation utt = a2 uxx .
5. Let f (x, y) be a continuous differentiable function. Analyze the level curves near a critical
value if that critical value is a max or a min. What if the level curve is a saddle point?
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