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Can anyone here help me do the following question? I tried isolating f(x) and f(y), but it doesn't really seem to go anywhere.

Q: Suppose f is a functionb which satisfies

f(x+y) = f(x) + f(y) +xy - x^3y + xy^3 - y^4 for all x, y, are elements of real numbers. Suppose, furthermore, that

lim f(x) / x = 1

x -> 0

a) Find f(0)

b) Show that f is differentiable at 0 (any tips)?

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# Finding f(0) for the following function

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