`cos(a + b) = cosa.cosb - sina.sinb `
`cos (a - b) = cosa.cosb + sina.sinb`
`sin(a + b) = sinacosb + cosa.sinb`
`sin(a - b) = sina.cosb - cosa.sinb`
`tan (a - b) =` `frac{tana - tanb}{1 + tana.tanb}`
`tan (a + b) `= `frac{tana + tanb}{1 - tana.tanb}`
`sin2a = 2sina.cosa`
`cos2a = cos²a - sin²a`
tan2a = `frac{2tana}{1 - tan²a}`