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JEE Advanced 2025 - Mathematics
22-01-2025 SHIFT-2
Question
Let $\alpha, \beta, \gamma$ and $\delta$ be the coefficients of $x^7, x^5, x^3$ and $x$ respectively in the expansion of $\left(x+\sqrt{x^3-1}\right)^5+\left(x-\sqrt{x^3-1}\right)^5, x>1$. If $u$ and $v$ satisfy the equations $\alpha u+\beta v=18, \gamma u+\delta v=20$, then $u+v$ equals :
Select the correct option:
A
8
B
3
C
5
D
4
✓ Correct! Well done.
✗ Incorrect. Try again or view the solution.
Solution
\begin{aligned} & \left(x+\sqrt{x^3-1}\right)^5+\left(x-\sqrt{x^3-1}\right)^5 \\ & =2\left\{{ }^5 C_0 \cdot x^5+{ }^5 C_2 \cdot x^3\left(x^3-1\right)+{ }^5 C_4 \cdot x\left(x^3-1\right)^2\right\} \\ & =2\left\{5 x^7+10 x^6+x^5-10 x^4-10 x^3+5 x\right\} \\ & \Rightarrow \alpha=10, \beta=2, \gamma=-20, \delta=10 \\ & \text { Now, } 10 \mathrm{u}+2 \mathrm{v}=18 \\ & -20 \mathrm{u}+10 \mathrm{v}=20 \\ & \Rightarrow \mathrm{u}=1, \mathrm{v}=4 \\ & \mathrm{u}+\mathrm{v}=5 \end{aligned}
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