Question

The value of $a$ for which the equations $3x-y+az$ $=1,2x+y+z=2,x+2y-az=-1$ fail to have unique solution is

• A. $7/2$
• B. $-7/2$
• C. $2/7$
• D. $-2/7$

Answer 1

The correct option is B $-7/2$

For fail to have unique solu.
$\left|\begin{array}{ccc}3& -1& a\\ 2& 1& 1\\ 1& 2& -a\end{array}\right|=0$
$3(-a-2)+1(-2a-1)+a(4-1)=0$
$-3a-6-2a-1+4a-a=0$
$2a+7=0$
$a=-7/2$