The solutions of given function;
(a) k = -12
(b) f'(3) = 0
(c) f'(x) = 0
Given info,
A function f is defined for all real numbers and has the following three properties:
[tex]f(1)=5 \quad f(3)=21 \quad f(a+b)-f(a)=k a b+2 b^{2}f(1)\\[/tex]
for all real numbers a and b where k is a fixed real number independent of a and b.
To find,
(a) Use a = 1 and b = 2 to find k.
[tex]f(a+b)-f(a)=k a b+2 b^{2}f(1)[/tex]
f(1 + 2) - f(1) = k(1 * 2) + 2 * 2² * f(1)
f(3) - f(1) = 2k + f(1) * 8
21 - 5 = 2k + 8 * 5
16 = 2k + 40
-2k = 24
k = -12
(b) Find f'(3).
f(3) = 21
f'(3) = d/dx(21) = 0
(c) Find f'(x) for all real x.
f'(x) = 0
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