- #1
Saladsamurai
- 3,020
- 7
Homework Statement
Given
[tex]A^{-1} =
\left[\begin{array} {cc}
2&-1\\
3&5
\end{array}\right]
[/tex]
find A.
The only thing that we have learned regarding inverses so far is that for a 2x2 matrix, the inverse is given by
[tex]A^{-1} = \frac{1}{ad-bc}
\left[\begin{array} {cc}
d&-b\\
-c&a
\end{array}\right]
[/tex]
So I set corresponding entries equal to each other and got the following 4 EQs:
[tex]a = \frac12-\frac{bc}{d}[/tex]
[tex]c = \frac{ad}{b}-1[/tex]
[tex]b = \frac13+\frac{ad}{b}[/tex]
[tex]d = \frac15 +\frac{ad}{c}[/tex]
So my question is, what is the best way to solve these? I have been trying substitution, but I guess I really don't know how to use that method with so many EQs?
I feel like I am just running in circles here! I took the 1st EQ and plugged the 2nd in for 'c' then I plugged the 3rd in for 'b' and the 4th for 'd.'
But that did not eliminate anything. I still have all 4 vars in the final EQ.
I need something more systematic. I am not sure which moves are legal here. How can eliminate something here?