Show the identity can be derived from a sum or a difference identity and the pythagorean identity?

cos(2a)=1-2sin^2acos(2a)=12sin2a

1 Answer
Jun 3, 2018

See explanation

Explanation:

Remember the angle sum identity

color(blue)(cos(x+y)=cos(x)cos(y)-sin(x)sin(y)cos(x+y)=cos(x)cos(y)sin(x)sin(y)

Now let color(red)(x=y=ax=y=a

cos(a+a)=cos(a)cos(a)-sin(a)sin(a)cos(a+a)=cos(a)cos(a)sin(a)sin(a)

=>cos(2a)=cos^2(a)-sin^2(a)cos(2a)=cos2(a)sin2(a)

By the pythagorean trig identity color(blue)(cos^2(a)=1-sin^2(a)cos2(a)=1sin2(a)

cos(2a)=(1-sin^2(a))-sin^2(a)cos(2a)=(1sin2(a))sin2(a)

=>cos(2a)=1-2sin^2(a) larr color(red)"What we wanted to show"cos(2a)=12sin2(a)What we wanted to show