Alan

That should be "on a pc" not "on a pic"!

I drew it in Mathcad (on a pic).

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The expression is a plane in 3D space:

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It's to do with PEMDAS.

\(\sqrt{\frac{1}{-1}}\rightarrow (\frac{1}{-1})^{1/2}\) 

Parentheses first, then exponent.

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These are exact values!  Presumably, you mean that, given they are angles, what are their values in degrees?

¶ radians = 180° so

1. 5¶/3 radians = 5*180/3 = 300°

etc.

f and g differ between -1<x<1:

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The mistake occurs in the first line:  \(\sqrt{\frac{1}{-1}}\ne\frac{\sqrt1}{\sqrt{-1}}\)  

Your subsequent development shows that equality here leads to an inconsistency.

We need to look at this as follows:  \(\sqrt{\frac{1}{-1}}\rightarrow\sqrt{(\frac{1}{-1})}\rightarrow\sqrt{-1}=i\)

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heureka's 126 should be 126x of course!

Try  \(g(x)=x^2+4\)  and  \(f(u)=\sqrt u\)

or  \(g(x)=x^2\)   and  \(f(u)=\sqrt{u+4}\)

or ...

"common"