Double Angle Formulas

Once you have memorized the addition formulas, there is no need to remember the double angle formulas as it's a derived version of addition formula.

By putting B equals to A in the addition formulas, we get

1) deriving double angle formulas

2)

To obtain the cos2 A , we make use of the trigonometry identity

3)

The double angle formulas are used regularly for proving trig identities and solving trigonometry equations . So it would be wise to memorise them well.

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