(sinx)4x+(cosx)4y=1x+y Then prove that (sinx)12x5+(cosx)12y5=1(x+y)5 ?

2 Answers
Sep 18, 2016

See below.

Explanation:

Solving for y

sin4(x)x+cos4(x)y=1x+y we obtain after some simplifications

y=x(cos(x)sin(x))2=xcot2(x)

Now, substituting back in

(sinx)12x5+(cosx)12y5=1(x+y)5

we can easily verify that the equality is observed.

Another way is making

y5sin12x+y5cos12x=(xyx+y)5 and using the previous result

ysin2x=xcos2xy5sin10x=x5cos10x

then

y5sin12x+y5sin10xcos2x=(xyx+y)5 so

y5sin10x=(xyx+y)5 so

sin10x=xx+xcos2xsin2x5 et voila!

Sep 19, 2016

GIVEN
sin4xx+cos4xy=1x+y

(x+yx)sin4x+(x+yy)cos4x=1

(1+yx)sin4x+(1+xy)cos4x=1

(sin2x+cos2x)22sin2xcos2x+(yx)sin4x+(xy)cos4x=1

12sin2xcos2x+(yx)sin4x+(xy)cos4x=1

2sin2xcos2x+(yx)sin4x+(xy)cos4x=0

(yxsin2xxycos2x)2=0

yxsin2x=xycos2x

sin2xx=cos2xy

By addendo

sin2xx=cos2xy=sin2x+cos2xx+y=1x+y

sin2x=xx+y

And

cos2x=yx+y

Now

sin12xx5+cos12xy5

=(sin2x)6x5+(cos2x)6y5

=(xx+y)6x5+(yx+y)6y5

=x(x+y)6+y(x+y)6

=1(x+y)5