Question

The sum of all values of $x$ for which the matrix $\left[\begin{array}{cc}3x& 2x\\ x+1& x-1\end{array}\right]$ is NOT invertible, is

[2 marks]

• A. $0$
• B. $1$
• C. $-1$
• D. $3$

Answer 1

The correct option is B $1$

For non-invertible :
$\left|\begin{array}{cc}3x& 2x\\ x+1& x-1\end{array}\right|=0$
$\Rightarrow 3x^2-3x-(2x^2+2x)=0$
$\Rightarrow x(x-1)=0$
$\Rightarrow x=0,1$
Required sum $=0+1=1$