Let $A=\left(\begin{array}{lll}1 & 0 & 0 \\ 2 & 1 & 0 \\ 3 & 2 & 1\end{array}\right)$. If $u_1$ and $u_2$…

Let $A=\left(\begin{array}{lll}1 & 0 & 0 \\ 2 & 1 & 0 \\ 3 & 2 & 1\end{array}\right)$. If $u_1$ and $u_2$ are column matrices such that $A u_1=\left(\begin{array}{l}1 \\ 0 \\ 0\end{array}\right)$ and $A u_2=\left(\begin{array}{l}0 \\ 1 \\ 0\end{array}\right)$, then $u_1+u_2$ is equal to
  1. $\left(\begin{array}{c}-1 \\ 1 \\ 0\end{array}\right)$
  2. $\left(\begin{array}{c}-1 \\ 1 \\ -1\end{array}\right)$
  3. $\left(\begin{array}{c}-1 \\ -1 \\ 0\end{array}\right)$
  4. $\left(\begin{array}{c}1 \\ -1 \\ -1\end{array}\right)$

Solution

$A=\left(\begin{array}{lll}1 & 0 & 0 \\ 2 & 1 & 0 \\ 3 & 2 & 1\end{array}\right)$ Let $u_1=\left[\begin{array}{l}a \\ b \\ c\end{array}\right] ; u_2=\left[\begin{array}{l}d \\ e \\ f\end{array}\right]$ $\mathrm{Au}_1=\left[\begin{array}{l}1 \\ 0 \\ 0\end{array}\right] \quad \Rightarrow u_1=\left[\begin{array}{c}1 \\ -2 \\ 1\end{array}\right]$ $\mathrm{Au}_2=\left[\begin{array}{l}0 \\ 1 \\ 0\end{array}\right] \quad \Rightarrow \mathrm{u}_2=\left[\begin{array}{c}0 \\ 1 \\ -2\end{array}\right] \quad \Rightarrow \mathrm{u}_1+\mathrm{u}_2=\left[\begin{array}{c}1 \\ -1 \\ -1\end{array}\right]$

Asked in: JEE Main 2012 (Offline)

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