Another funny function

Calculus Level 2

Consider a function f f satisfying f ( 1 ) = 2 f(1)=2 and f ( 1 ) = 3 f'(1)=3 . And let g ( x ) = ln ( f ( e x ) ) g(x)=\ln(f(e^x)) . Calculate the value of g ( 0 ) g'(0) .


The answer is 1.5.

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2 solutions

Discussions for this problem are now closed

Using the chain rule, we have that

g ( x ) = 1 f ( e x ) f ( e x ) d d x ( e x ) g'(x) = \dfrac{1}{f(e^{x})} * f'(e^{x}) * \dfrac{d}{dx}(e^{x}) .

Thus g ( 0 ) = 1 f ( 1 ) f ( 1 ) e 0 = 1 2 3 1 = 3 2 = 1.5 g'(0) = \dfrac{1}{f(1)} * f'(1) * e^{0} = \dfrac{1}{2} * 3 * 1 = \dfrac{3}{2} = \boxed{1.5} .

Moderator note:

Nicely done! One should also note that we don't need to use the value of f ( 1 ) f(1) because it doesn't matter in the calculation.

Jacob Heyn
Nov 3, 2014

The first step is solving for f(x).

We know the slope of f is equal to 3.

f(x)=3x+b

We also know that the f(1)=2

2=3(1)+b=2-3=b

b=-1

f(x)=3x-1

We then find the f(e^x) by substituting e^x for x.

f(e^x)=3e^x-1

We then plug this equation in for the f(e^x).

g(x)=ln(3e^x-1)

We then must use chain rule to get the derivative.

The derivative for ln(u) where u is another equation is (1/u)(u')

g'(x)=(3e^x)/(3e^x-1)

g'(0)=(3e^0)/(3e^0-1)=3/(3-1)=3/2=1.5

Moderator note:

This solution has been marked wrong. You shouldn't assume that the function f f is linear simply because you are only given the first derivative of f f .

Nice solution Can you please explain how you concluded that function is linear?????? for example the function f ( x ) = x 2 x + 2 f(x)=x^2-x+2 ..can also satisfy given conditions

Aman Sharma - 6 years, 7 months ago

the function is not linear. the derivative is a linear operator though

Jim Adriazola - 6 years, 5 months ago

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