The following particularly the first of the three below areed "Pythagorean" iden.ies. You can see the PythagoreanThereom relationship clearly if you consider the unit circle, where the angle is t, the "opposite" side is sin t = y, the "adjacent" side is cos t = x, and the hypotenuse is 1..
Cos  = cos cos + sin sin By the way, in the above iden.ies, the angles are denoted by Greek letters. The atype letter, "", ised "alpha", which isounced "ALfuh".. Ptolemy's iden.ies, the sum and difference formulas for sine and cosine. Double angle formulas for sine and cosine. Note that there are three forms for the double angle formula for cosine. You only need to know one, but be able to derive the other two from the Pythagorean formula..Pythagorean iden.ies. In tr.nometry, the basic relationship between the sine and the cosine is known as one of the Pythagorean iden.y: where sin2 means sin 2 and cos2 means cos 2. This can be viewed as a version of the Pythagorean theorem, and follows from the equation x2 + y2 = 1 for the unit circle..Cos A cos B 2 sin A + B sin A B In the proofs, the student will see that the iden.ies e through h are inversions of a through d respectively, which are proved first..
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Iden Y Mathematics Wikipedia
In mathematics an iden.y is an equality relation A = B, such that A and B contain some variables and A and B produce the same value as each other regardless of what .

Eulers Iden Y Wikipedia
Euler's iden.y is a special case of Euler's formula from complexysis, which states that for any real number x, = + where the inputs of the .

Openid Foundation Website
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Mathwords Iden Y
Iden.y Equation or Inequality An equation which is true regardless of what values are subs.uted for any variables if there are any variables .