Gujarati
3 and 4 .Determinants and Matrices
normal

The total number of distinct $x \in \mathbb{R}$ for which $\left|\begin{array}{ccc}x & x^2 & 1+x^3 \\ 2 x & 4 x^2 & 1+8 x^3 \\ 3 x & 9 x^2 & 1+27 x^3\end{array}\right|=10$ is

A

$2$

B

$3$

C

$5$

D

$4$

(IIT-2016)

Solution

$\left[\begin{array}{ccc} x & x ^2 & 1+ x ^3 \\ 2 x & 4 x ^2 & 1+8 x ^3 \\ 3 x & 9 x ^2 & 1+27 x ^3\end{array}\right]=10$

$\Rightarrow x^3 \times \operatorname{det}\left[\begin{array}{ccc}1 & 1 & 1+x^3 \\ 2 & 4 & 1+8 x^3 \\ 3 & 9 & 1+27 x^3\end{array}\right]=10$

$\Rightarrow x^3 \times\left[4+108 x^3-9-72 x^3-2-54 x^3+3+24 x^3+6+6 x^3\right]=10$

$\Rightarrow x^3\left(12 x^3+2\right)=10$

$\Rightarrow 6 x^6+x^3-5=0$

$\Rightarrow 6 x^6+6 x^3-5 x^3-5=0$

$\Rightarrow\left(x^3+1\right)\left(6 x^3-5\right)=0$

$\Rightarrow x^3=-1, \frac{5}{6}$, both of which have one real root respectively.

Hence, total number of distinct values of $x$ is $2$ .

Standard 12
Mathematics

Similar Questions

Start a Free Trial Now

Confusing about what to choose? Our team will schedule a demo shortly.