Instead, you should have divided by 3:R2=R2/3-R1/3

[Airbus]

Homework Statement

A = \left(\begin{array}{cccc}<br /> 3 &amp; 5 &amp; -2 &amp; 7\\<br /> 1 &amp; 2 &amp; -1 &amp; 1\\<br /> 2 &amp; 4 &amp; 1 &amp; 5\\<br /> 3 &amp; 7 &amp; 5 &amp; 3<br /> \end{array}<br /> \right)

Homework Equations

The Attempt at a Solution

R4=R4-R1
R3=R3-2R2
A = \left(\begin{array}{cccc}<br /> 3 &amp; 5 &amp; -2 &amp; 7\\<br /> 1 &amp; 2 &amp; -1 &amp; 1\\<br /> 0 &amp; 0 &amp; 3 &amp; 3\\<br /> 0 &amp; 2 &amp; 7 &amp; -4<br /> \end{array}<br /> \right)
R2=3R2-R1
A = \left(\begin{array}{cccc}<br /> 3 &amp; 5 &amp; -2 &amp; 7\\<br /> 0 &amp; 1 &amp; -1 &amp; 4\\<br /> 0 &amp; 0 &amp; 3 &amp; 3\\<br /> 0 &amp; 2 &amp; 7 &amp; -4<br /> \end{array}<br /> \right)
R4=R4-2R2
A = \left(\begin{array}{cccc}<br /> 3 &amp; 5 &amp; -2 &amp; 7\\<br /> 0 &amp; 1 &amp; -1 &amp; 4\\<br /> 0 &amp; 0 &amp; 3 &amp; 3\\<br /> 0 &amp; 0 &amp; 9 &amp; 4<br /> \end{array}<br /> \right)
R4=R4-3R3
A = \left(\begin{array}{cccc}<br /> 3 &amp; 5 &amp; -2 &amp; 7\\<br /> 0 &amp; 1 &amp; -1 &amp; 4\\<br /> 0 &amp; 0 &amp; 3 &amp; 3\\<br /> 0 &amp; 0 &amp; 0 &amp; -5<br /> \end{array}<br /> \right)

3*1*3*-5 = -45

but the ans. is -15

Can someone correct me?
Because I couldn't find any mistake

Thank you very much

 

[jbunniii]

You multiplied the determinant by 3 when you did this operation:

R2=3R2-R1