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