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单词 WeierstrassSubstitutionFormulas
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Weierstrass substitution formulas


The Weierstrass substitution formulas for -π<x<π are:

sinx=2t1+t2cosx=1-t21+t2dx=21+t2dt

They can be obtained in the following manner:

Make the Weierstrass substitution t=tan(x2). (This substitution is also known as the universal trigonometric substitution.) Then we have

cos(x2)=1sec(x2)=11+tan2(x2)=11+t2

and

sin(x2)=cos(x2)tan(x2)=t1+t2.

Note that these are just the “formulas involving radicalsMathworldPlanetmath (http://planetmath.org/Radical6)” as designated in the entry goniometric formulasPlanetmathPlanetmath; however, due to the restriction on x, the ±’s are unnecessary.

Using the above formulas along with the double angle formulas, we obtain

sinx=2sin(x2)cos(x2)=2t1+t211+t2=2t1+t2

and

cosx=cos2(x2)-sin2(x2)=(11+t2)2-(t1+t2)2=11+t2-t21+t2=1-t21+t2.

Finally, since t=tan(x2), solving for x yields that x=2arctant. Thus, dx=21+t2dt.

The Weierstrass substitution formulas are most useful for integrating rational functions of sine and cosine (http://planetmath.org/IntegrationOfRationalFunctionOfSineAndCosine).

TitleWeierstrass substitution formulas
Canonical nameWeierstrassSubstitutionFormulas
Date of creation2013-03-22 17:05:25
Last modified on2013-03-22 17:05:25
OwnerWkbj79 (1863)
Last modified byWkbj79 (1863)
Numerical id12
AuthorWkbj79 (1863)
Entry typeDefinition
Classificationmsc 26A36
Classificationmsc 33B10
SynonymWeierstraß substitution formulas
Related topicGoniometricFormulae
Related topicIntegrationOfRationalFunctionOfSineAndCosine
Related topicPolynomialAnalogonForFermatsLastTheorem
DefinesWeierstrass substitution
DefinesWeierstaß substitution
Definesuniversal trigonometric substitution
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