# Math Help - Inverse fxn help

1. ## Inverse fxn help

Let f(x)=(x^4)+(x^3)+1
Let g(x) be the inverse of f(x) and define F(x)=f(2g(x)). Find an equation for the tangent line to y=F(x) at x=3.

2. ## Re: Inverse fxn help

Hint: $g(3)=1$ and $g'(3)=1/f'(1)$ . Now, apply the Chain's Rule.

3. ## Re: Inverse fxn help

hello there
in order to compute the tangent line u need
the value of the function f(2g(x)) at x=3
and the value of the derivative of the function
at the same point meaning F'(3)
first u have to find f(2g(x))
just substitute in f(x) 2g(x)
u get
$16g(x)^4+8g(x)^3+1$
now what we need is actually
$16g(3)^4+8g(3)^3+1$
to know the value of this expression we need
to know the value of g(3)
since g(x) is the inverse of f(x)
then f(g(3))=3
in other words:
$g(3)^4+g(3)^3+1=3$
$g(3)^4+g(3)^3-2=0$
by guessing we get that the solution is:
g(3)=1
(there is another solution but its hard to get
it actually we need approximation)
now that we have g(3)=1
then F(3)=16*1^4+8*1^3+1=25
NOW we have to know the value of
F'(3)(the derivative)
$F'(3)=4*16*g(3)^3*g'(3)+3*8*g(3)^2*g'(3)$
to now the value of F'(3) we have to
know the value of g'(3) since we already know the value
of g(3)
now since f(g(x)=x
if we derive both sides using the chain rule
we get
$f'(g(x)*g'(x)=1$
now divide both sides by f'(g(x))
$g'(x)=1/f'(g(x)$
$g'(x)=1/f'(g(x)$
$g'(x)=1/(4g(x)^3+3g(x)^2)$
now just subsitute x=3
u get
g'(3)=1/7
can u proceed from here??

4. ## Re: Inverse fxn help

Originally Posted by islam
hello there
in order to compute the tangent line u need
the value of the function f(2g(x)) at x=3
and the value of the derivative of the function
at the same point meaning F'(3)
first u have to find f(2g(x))
just substitute in f(x) 2g(x)
u get
$16g(x)^4+8g(x)^3+1$
now what we need is actually
$16g(3)^4+8g(3)^3+1$
to know the value of this expression we need
to know the value of g(3)
since g(x) is the inverse of f(x)
then f(g(3))=3
in other words:
$g(3)^4+g(3)^3+1=3$
$g(3)^4+g(3)^3-2=0$
by guessing we get that the solution is:
g(3)=1
(there is another solution but its hard to get
it actually we need approximation)
now that we have g(3)=1
then F(3)=16*1^4+8*1^3+1=25
NOW we have to know the value of
F'(3)(the derivative)
$F'(3)=4*16*g(3)^3*g'(3)+3*8*g(3)^2*g'(3)$
to now the value of F'(3) we have to
know the value of g'(3) since we already know the value
of g(3)
now since f(g(x)=x
if we derive both sides using the chain rule
we get
$f'(g(x)*g'(x)=1$
now divide both sides by f'(g(x))
$g'(x)=1/f'(g(x)$
$g'(x)=1/f'(g(x)$
$g'(x)=1/(4g(x)^3+3g(x)^2)$
now just subsitute x=3
u get
g'(3)=1/7
can u proceed from here??
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