The double angle formulas are used just like the other trigonomic formulas. You must first identify it as a double angle which is really easy. One way is that there will me the function 2 then theta (@). Most of the time however you will be asked to simplify it. Then you will just take the information and plug it into the other half of the formula.
sin2@=2sin@cos@
cos2@=cos^2@-sin^2
cos2@=1-2sin^2
cos2@=2cos^2@-1
tan2@=2tan@/1-tan^2@
Ex1. Simplify cos^2(30)-sin^2(30)
A.First idenitify formula. cos2@=cos^2@-sin^2@
B.Then plug in. cos2(30)
C.Then solve. cos60=squareroot of 3/2 (From trig chart)
Done!!!
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