Réduction de degré
`sin^2α=(1-cos2α)/2`
`cos^2α=(1+cos2α)/2`
`tan^2α=(1-cos2α)/(1+cos2α)`
`cot^2α=(1+cos2α)/(1-cos2α)`
`sin^3α=(3sinα-sin3α)/4`
`cos^3α=(3cosα-cos3α)/4`
`tan^3α=(3sinα-sin3α)/(3cosα + cos3α)`
`cot^3α=(3cosα+cos3α)/(3sinα - sin3α)`
`sin^4α=(3-4cos2α + cos4α)/8`
`cos^4α=(3+4cos2α + cos4α)/8`
`sin^5α=(10sinα-5sin3α+sin5α)/16`
`cos^5α=(10cosα+5cos3α+cos5α)/16`