# Matrices det (a)=0

• Nov 29th 2012, 10:29 AM
Petrus
Matrices det (a)=0
The question is that i shall find all numbers for a so that det (A)=0
i basicly dont know what trick i shall use!
edit: i did start to det it and set it to zero but i get wrong
• Nov 29th 2012, 11:11 AM
Scopur
Re: Matrices det (a)=0
First find the determinant using cofactor expansion method.
• Nov 29th 2012, 11:24 AM
Petrus
Re: Matrices det (a)=0
yes i did it.. i get -2a^2+10a-20 and then i set it to zero and get the answer
a=25
a=-10
the answe shall be 3 and 2 but idk what i do wrong =S
• Nov 29th 2012, 11:25 AM
Plato
Re: Matrices det (a)=0
Quote:

Originally Posted by Petrus
The question is that i shall find all numbers for a so that det (A)=0
i basicly dont know what trick i shall use!
edit: i did start to det it and set it to zero but i get wrong

Expand these and solve for a.

$2\left| {\begin{array}{rr} a & 1 \\ 2 & {2a + 2} \\ \end{array} } \right| - \left| {\begin{array}{rr} { - 2} & 1 \\ {2a} & 2 \\ \end{array} } \right| + 3\left| {\begin{array}{rr} { - 2} & a \\ a & 2 \\ \end{array} } \right| = 0$
• Nov 29th 2012, 11:30 AM
Petrus
Re: Matrices det (a)=0
Quote:

Originally Posted by Plato
Expand these and solve for a.

$2\left| {\begin{array}{rr} a & 1 \\ 2 & {2a + 2} \\ \end{array} } \right| - \left| {\begin{array}{rr} { - 2} & 1 \\ {2a} & 2 \\ \end{array} } \right| + 3\left| {\begin{array}{rr} { - 2} & a \\ a & 2 \\ \end{array} } \right| = 0$

so im basicly pretty new on matrice :P ik how to expaned but why did u do it? and why cant we use sorrow rule or what the name to find det on 3x3 matrice?
• Nov 29th 2012, 11:48 AM
Plato
Re: Matrices det (a)=0
Quote:

Originally Posted by Petrus
so im basicly pretty new on matrice :P ik how to expaned but why did u do it? and why cant we use sorrow rule or what the name to find det on 3x3 matrice?

I have no idea what the meaning of that may be.

You should get $-2a^2+10a-12$ and answers $2,~3$.