# easy angle thing

• Jan 25th 2011, 09:43 PM
furor celtica
easy angle thing
ok so i have this list of questions like this one:
find the least positive value of B for which sin(A-B)=cos(A+B)

using translation, odd, periodic properties, etc.
the problem is i totally forgot how to work these out! i know there is a simple method to work this out without staring continually at the sin and cos graphs, i just need a little reminder. thanks
• Jan 26th 2011, 02:10 AM
Sudharaka
Quote:

Originally Posted by furor celtica
ok so i have this list of questions like this one:
find the least positive value of B for which sin(A-B)=cos(A+B)

using translation, odd, periodic properties, etc.
the problem is i totally forgot how to work these out! i know there is a simple method to work this out without staring continually at the sin and cos graphs, i just need a little reminder. thanks

Dear furor celtica,

$\sin(A-B)=\cos(A+B)$

$\cos\left(\frac{\pi}{2}-(A-B)\right)=\cos(A+B)$

$\frac{\pi}{2}-(A-B)=2n\pi\pm(A+B)\mbox{ where }n\in Z$

$A=\frac{\pi}{4}-n\pi\mbox{ or }B=n\pi-\frac{\pi}{4}\mbox{ where }n\in Z$

Therefore the smallest positive value for B would be,

$n=1\Rightarrow{B=\pi-\dfrac{\pi}{4}=\dfrac{3\pi}{4}}$

Hope you understood.
• Jan 26th 2011, 04:56 AM
furor celtica
umm i got most of it but at the level i'm at the n is not really appropriate, could you kinda simplify that?
• Jan 26th 2011, 05:47 AM
Sudharaka
Quote:

Originally Posted by furor celtica
umm i got most of it but at the level i'm at the n is not really appropriate, could you kinda simplify that?

Dear furor celtica,

Do you mean that you do not understand how we got the n into the equation?