Question

Let g be the function defined by g(x) = x3 + x. If f(x) = g-1(x) and f(2) = 1, what is the value of f ′ (2)?

Answer 1

f'(2) = 1/4

Explanation:

We know that:

g(x) = x^3 + x

and f(x) is the inverse function of g(x), such that:

f(2) = 1

this is because:

g(1) = 1^3 + 1 = 2

We want to find:

f'(2)

The general formula for this case is:

if f(x) is the inverse of g(x)

and f(x) = y

then:

f'(y) = 1/g'(x)

Then in this case,

f'(2) = 1/g'(1)

so we just need to differentiate g(x)

g'(x) = 3*x + 1

and:

g'(1) = 3*1 + 1 = 4

Then:

f'(2) = 1/g'(1) = 1/4

f'(2) = 1/4