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

[Airbus]

Homework Statement

$$A = \left(\begin{array}{cccc} 3 & 5 & -2 & 7\\ 1 & 2 & -1 & 1\\ 2 & 4 & 1 & 5\\ 3 & 7 & 5 & 3 \end{array} \right)$$

Homework Equations

The Attempt at a Solution

R4=R4-R1
R3=R3-2R2
$$A = \left(\begin{array}{cccc} 3 & 5 & -2 & 7\\ 1 & 2 & -1 & 1\\ 0 & 0 & 3 & 3\\ 0 & 2 & 7 & -4 \end{array} \right)$$
R2=3R2-R1
$$A = \left(\begin{array}{cccc} 3 & 5 & -2 & 7\\ 0 & 1 & -1 & 4\\ 0 & 0 & 3 & 3\\ 0 & 2 & 7 & -4 \end{array} \right)$$
R4=R4-2R2
$$A = \left(\begin{array}{cccc} 3 & 5 & -2 & 7\\ 0 & 1 & -1 & 4\\ 0 & 0 & 3 & 3\\ 0 & 0 & 9 & 4 \end{array} \right)$$
R4=R4-3R3
$$A = \left(\begin{array}{cccc} 3 & 5 & -2 & 7\\ 0 & 1 & -1 & 4\\ 0 & 0 & 3 & 3\\ 0 & 0 & 0 & -5 \end{array} \right)$$

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