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September 17, 2014

Posted by **Suzy** on Sunday, November 28, 2010 at 5:30pm.

f(x) = (1 + x)(1 + x4)(1 + x16)(1 + x64)(1 + x256) · · ·

Compute f to the power of -1 times 8 divided by 5 times f times 3/8

- Math -
**MathMate**, Sunday, November 28, 2010 at 9:39pmFirst, we have to establish that the function f(x) is invertible on the interval [0,1].

f(x) consists of a polynomial with all positive terms, so it is strictly increasing and consequently one-to-one and onto. Thus f^{-1}(x) exists.

Where x can be solved explicitly for y, an analytic expression of the inverse can be found. In other cases, we can resort to numerical solutions, which can be obtained to any accuracy we wish. For the given problem, we will supply a numerical solution.

We start with a property of f^{-1}(x). which can be looked at as

f^{-1}(f(x)) = x for all x on the given interval.

Thus f^{-1}(f(3/8))= 3/8.

To find y=f^{-1}(8/5f(3/8)), we need to find y for which

f(y)=8/5f(3/8)

then

f^{-1}(f(y))

=f^{-1}(85f(3/8))

=y

We can find the approximation by the secant method.

First evaluate

8/5f(3/8)

=2.243506202480352

Knowing f(0)=1, f(1)=32,

we give a first approximation of

y0=0.7

f(0.7)=2.115

f(0.8)=2.609

The new y could be interpolated:

y1=0.7+(0.8-0.7)*(2.2435-2.115)/(2.609-2.115)

=0.72602

We get successively

y2=0.73172

y3=0.73179

....

y=0.7317902526225

Check:

f(y)=2.243506202480352 OK

Therefore:

f^{-1}(8/5f(3/8))=0.7317902526225

- Math -
**Freaky**, Sunday, November 28, 2010 at 11:48pmIDK!!!

but the answer above needs to be put in simpler words to understand it

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