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## Help me find fx from gx,f(fx) ?

Asked by Nguyen Hoang Thien Phuoc

### Nguyen Hoang Thien Phuoc (view profile)

on 4 Jan 2013

Find fx from gx and f(gx)

Ex:

find f(1/2) with gx=1-x^2 and f(gx)=(x^4+x^2)/(1+x^2)

```syms x;
a=input(' a= ');
g=input(' gx= ');
f=input(' fx = ');
```

...?????? I don't know...please help me ! Thanks a lot!

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## 1 Answer

Answer by bym

### bym (view profile)

on 5 Jan 2013

here is a start

```clc;clear
syms x
gx = input('gx = ','s');
fx = input('f(gx) = ','s');
a = input('f(n) = ','s');  % 1/2 in your example
```
```n = solve([gx '-' a],'x')
```
```% f(1/2) = ? for you to complete
doc subs
```

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