Show the identity can be derived from a sum or a difference identity and the pythagorean identity?
cos(2a)=1-2sin^2acos(2a)=1−2sin2a
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
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
cos(2a)=(1-sin^2(a))-sin^2(a)cos(2a)=(1−sin2(a))−sin2(a)
=>cos(2a)=1-2sin^2(a) larr color(red)"What we wanted to show"⇒cos(2a)=1−2sin2(a)←What we wanted to show