Question #347333

If f(x) = sin x, show that f (2x)= 2f(x) f(1/2 π-x).


1
Expert's answer
2022-06-06T23:03:55-0400

If f(x)=sinx,f(x)=sinx, then

f(2x)=sin2x=2sinxcosxf(2x)=sin2x=2sinxcosx (double angle formula).


Let's consider the right side of the equality:

2f(x)f(π2x)=2sinxsin(π2x)=2sinxcosx,2f(x)f(\frac{\pi}{2}-x)=2sinxsin(\frac{\pi}{2}-x)=2sinxcos x,

because sin(π2x)=cosxsin(\frac{\pi}{2}-x)=cosx (co-function identity).


So we have:

f(2x)=2sinxcosx=2f(x)f(π2x),f(2x)=2sinxcosx=2f(x)f(\frac{\pi}{2}-x),

and the statement is proved.


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