# Euler's Formula

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## Euler's formula

[′ȯi·lərz ‚fȯr·myə·lə]
(mathematics)
The formula e ix = cos x + i sin x, where i = √(-1).

## Euler’s Formula

any of several important formulas established by L. Euler.

(1) A formula giving the relation between the exponential function and trigonometric functions (1743):

eix = cos x + i sin x

Also known as Euler’s formulas are the equations

(2) A formula giving the expansion of the function sin x in an infinite product (1740):

(3) The formula

where s = 1,2,... and p runs over all prime numbers.

(4) The formula

(a2 + b2 + c2 + d2)(p2 + q2 + r2 + s2) = x2 + y2 + z2 + t2

where

x = ap + bq + cr + ds

y = aqbp ± csdr

z = arbscp ± dq

t = as ± brcqdp

(5) The formula (1760)

Also known as the equation of Euler, it gives an expression for the curvature 1/R of a normal section of a surface in terms of the surface’s principal curvatures 1/R1 and 1/R2 and the angle φ between one of the principal directions and the given direction.

Other well-known formulas associated with Euler include the Euler-Maclaurin summation formula and the Euler-Fourier formulas for the coefficients of expansions of functions in trigonometric series.

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