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The cosine can be expressed with complex exponential functions as



In the power series for the cosine are only even numbers

so we can use the imaginary unit to give all terms a plus sign

After duplicating all terms you get

To this we add odd exponents and subtract these immedeiatelyweer van af

Reshuffling gives

In braces are two power series for exponential functions and thus

so that


Example 1

You can see that cos (½π) = 0, because


Example 2

You can see that cos (0) = cos (2π) = cos (4π) = 1, because


Example 3

You can see that cos (π) = cos (3π) = −1, because



This formula for the cosine was described by the Swiss mathematician Leonhard Euler (1707 - 1783) .

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