Differential Equations Help!
Undetermined coefficients
y''+3y=-9

Question

Differential Equations Help! 
Undetermined coefficients
 y''+3y=-9

abb0t · Answer

start by finding the complimentary solution first, then your particular.

zzr0ck3r · Answer

what is the characteristic equation?

anonymous · Answer

is my general solution 
$$y=C1Cos(2\sqrt{3}x)+C2Sin(2\sqrt{3}x)$$

abb0t · Answer

$r^2+3 = 0$ $\Rightarrow$ $r= \pm \sqrt{3}i$
so,

anonymous · Answer

oh damn I noob'd it and used -9 :/

abb0t · Answer

wait, how did you get $2\sqrt{3}$???

abb0t · Answer

anyways, i THINK they say to use a "guess" for your constant, but I am more familiar with the annihilator method, which is somewhat the same thing.

anonymous · Answer

instead of r^2+3=0 I did r^2+3=-9

anonymous · Answer

yeah my bad thanks though cause I probably would've never found it

abb0t · Answer

oh. ok. so you're ok from here or did you still need more help?

anonymous · Answer

im gonna try to work it if you'd like to give me a few min ill put up what I get or where I get stuck

anonymous · Answer

does A=-3?

anonymous · Answer

@abb0t if you get a chance to check my answer it would be greatly appreciated!

abb0t · Answer

right.

anonymous · Answer

beast! thanks