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### Tautology

A calculation is a tautology. In mathematics, this means that at the left and right of the equal sign (=) the same is stated.

##### Example 1

Some simple calculations show

If left and right are not identical you have made a mistake. That is it.

##### Example 2

You can solve the equation

as for *x = *2 it gives

2

^{2}= 2^{2}⇒ 4 = 4

and that is correct. You can easily see that. But there are more solutions. By trial and error you can find them.

##### Example 3

Substituting the definitions of the sine and the cosine in Euler's formula gives