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JEE MAIN 2025
23-01-2025 SHIFT-2
Question
Let the point A divide the line segment joining the points $P(-1,-1,2)$ and $Q(5,5,10)$ internally in the ratio $r: 1(r>0)$. If $O$ is the origin and $(\overrightarrow{O Q} \cdot \overrightarrow{O A})-\frac{1}{5}|\overrightarrow{O P} \times \overrightarrow{O A}|^{2}=10$, then the value of $r$ is :
Select the correct option:
A
7
B
14
C
3
D
$\sqrt{7}$
✓ Correct! Well done.
✗ Incorrect. Try again or view the solution.
Solution
$A \equiv\left(\frac{5 r-1}{r+1}, \frac{5 r-1}{r+1}, \frac{10 r+2}{r+1}\right)$
$(\overrightarrow{\mathrm{OQ}} \cdot \overrightarrow{\mathrm{OA}})-\frac{|\overrightarrow{\mathrm{OP}} \times \overrightarrow{\mathrm{OA}}|^{2}}{5}=10$
$\overrightarrow{\mathrm{OQ}} \cdot \overrightarrow{\mathrm{OA}}=\frac{10}{\mathrm{r}+1}(15 \mathrm{r}+1)$
$|\overrightarrow{\mathrm{OP}} \times \overrightarrow{\mathrm{OA}}|^{2}=\frac{\mathrm{r}^{2}}{(\mathrm{r}+1)^{2}}(800)$
so by equation (1)
$\frac{10}{r+1}(15 r+1)-\frac{1}{5} \frac{r^{2}(800)}{(r+1)^{2}}=10$
$2 r^{2}-14 r=0$
$r=7, r \neq 0$
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