# Trignometry qus

• Aug 31st 2007, 08:42 PM
tom_asian
Trignometry qus
Find all values of x in the interval 0≤X2 pie

a) sin x = cos x

Could you please provide me working for it, thanks.
• Aug 31st 2007, 08:48 PM
tom_asian
Sorry, dont worry about working the qus out..

I just figured out how to do it, thanks
• Aug 31st 2007, 08:50 PM
Jhevon
Quote:

Originally Posted by tom_asian
Find all values of x in the interval 0≤X2 pie

a) sin x = cos x

Could you please provide me working for it, thanks.

well, you should pretty much know by heart that $\sin x = \cos x$ for $x = \frac {\pi}{4} + k \pi$

but if you wanted to work it out explicitly, here's what to do

$\sin x = \cos x$

$\Rightarrow \sin^2 x = \cos^2 x$

$\Rightarrow \cos^2 x - \sin^2 x = 0$

$\Rightarrow \cos 2x = 0$ ........you should also know for what values $\cos x = 0$

$\Rightarrow 2x = \frac {\pi}{2} + 2k \pi$

$\Rightarrow x = \frac {\pi}{4} + k \pi$

Now i leave the easy part for you. what are the values in the interval we were given?
• Aug 31st 2007, 08:51 PM
Jhevon
Quote:

Originally Posted by tom_asian
Sorry, dont worry about working the qus out..

I just figured out how to do it, thanks

couldn't you have said that sooner? :p
• Sep 1st 2007, 12:01 AM
topsquark
Quote:

Originally Posted by Jhevon
well, you should pretty much know by heart that $\sin x = \cos x$ for $x = \frac {\pi}{4} + k \pi$

but if you wanted to work it out explicitly, here's what to do

$\sin x = \cos x$

$\Rightarrow \sin^2 x = \cos^2 x$

$\Rightarrow \cos^2 x - \sin^2 x = 0$

$\Rightarrow \cos 2x = 0$ ........you should also know for what values $\cos x = 0$

$\Rightarrow 2x = \frac {\pi}{2} + 2k \pi$

$\Rightarrow x = \frac {\pi}{4} + k \pi$

Now i leave the easy part for you. what are the values in the interval we were given?

Ah! But $cos(2x) = 0$ for
$x = \frac{ \pi}{2} + 2k \pi \text{ and } \frac{3 \pi}{2} + 2k \pi$

So
$x = \frac{ \pi}{4} + k \pi \text{ and } \frac{3 \pi}{4} + k \pi$

It is just that the $3 \pi /4 + k \pi$ solution is extraneous in this case, its origin coming from squaring both sides of the original equation.

-Dan
• Sep 1st 2007, 05:28 PM
ThePerfectHacker
And easier solution.
$\sin x = \cos x$
Divide by $\cos x$ to get,
$\tan x = 1$ now continue...