# trigonometry

• Dec 31st 2008, 08:47 PM
Singular
trigonometry
How to proof ,

$
(\sin x + \cos x)\sin 2x = \sqrt 2
$

thanks
• Dec 31st 2008, 09:03 PM
Krizalid
You probably meant to say "how to solve" instead "how to prove," since as it's written, it just holds for certain values, in fact, it's a trig. equation.
• Dec 31st 2008, 09:05 PM
Last_Singularity
Quote:

Originally Posted by Singular
How to proof ,

$
(\sin x + \cos x)\sin 2x = \sqrt 2
$

thanks

$(\sin x + \cos x)\sin 2x = \sqrt 2$ cannot be proved because it is not true. I think that what you meant to ask is how to solve that equation for $x$

Try squaring both sides and expanding:
$(\sin x + \cos x)^2 \sin^2 2x = 2$
$(\sin^2 x + 2 \cos x \sin x + \cos^2 x) \sin^2 2x = 2$
$(1+ 2 \cos x \sin x) \sin^2 2x = 2$
$(1+ \sin 2x) \sin^2 2x = 2$

Letting $y = \sin 2x$, you have
$(1+ \sin 2x) \sin^2 2x = 2 \longrightarrow (1+y)y^2 = 2$

Solve for y by inspection. I see one obvious value of y such that $(1+y)y^2 = 2$

Then find when $\sin 2x$ equals that value of y and you're done.

Note: there is more than one solution. In fact, there are infinitely many solutions to this problem because sine is a periodic function.
• Dec 31st 2008, 09:06 PM
Singular
owh... my mistake... sorry