# Solve System of Equations with A^-1

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• December 7th 2012, 10:03 AM
bustacap09
Solve System of Equations with A^-1
Write the system in the vector matrix form Ax=b and solve by finding A^-1

Vector Matrix form?
[2,1;4,3]*[x;y] = [2,-4]

2x+y=2
4x+3x=-4

So A^-1 =
[1.5,-.5;-2,1]

So I don't understand what I am suppose to do with the A^-1. I understand how to use ref, rref and Cramer's rule to solve these equations but I am not understanding what method they are wanting me to use.
Reference: Answer is x=5, y=-8
• December 7th 2012, 10:20 AM
topsquark
Re: Solve System of Equations with A^-1
Quote:

Originally Posted by bustacap09
Write the system in the vector matrix form Ax=b and solve by finding A^-1

Vector Matrix form?
[2,1;4,3]*[x;y] = [2,-4]

2x+y=2
4x+3x=-4

So A^-1 =
[1.5,-.5;-2,1]

So I don't understand what I am suppose to do with the A^-1. I understand how to use ref, rref and Cramer's rule to solve these equations but I am not understanding what method they are wanting me to use.
Reference: Answer is x=5, y=-8

Well you have Au = v. You found A^{-1}, so then A^{-1}Au = A^{-1}v implies u = A^{-1}v.

Does that answer the question or am I not getting where you are confused?

-Dan
• December 7th 2012, 11:06 AM
bustacap09
Re: Solve System of Equations with A^-1
"Well you have Au = v. You found A^{-1}, so then A^{-1}Au = A^{-1}v implies u = A^{-1}v."

I didn't understand how you used the A^-1 to find the values for [x,y]. But this makes perfect sense.

Thank you very much.
• December 7th 2012, 04:02 PM
ANDS!
Re: Solve System of Equations with A^-1
If $A\overline{x}=\overline{b}$, then $\overline{x}=A^{-1}\overline{b}$. So, $\left(\begin{matrix}{\frac{3}{2} & -\frac{1}{2}\\-2 & 1\end{matrix}\right)\left(\begin{matrix}2\\-4\end{matrix}\right)=\left(\begin{matrix}5\\-8\end{matrix}\right)$. Straight up plug and chuggin'.