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← JEE Main 2022 · 26 Jun · Shift 2
MathematicsInteger+4 / -1Question 24 of 90

JEE Main 2022 · 26 Jun · Shift 2, Mathematics question 24

Let X=[010001000]X=\begin{bmatrix}0&1&0\\0&0&1\\0&0&0\end{bmatrix}, Y=αI+βX+γX2Y=\alpha I+\beta X+\gamma X^2 and Z=α2I−αβX+(β2−αγ)X2Z=\alpha^2I-\alpha\beta X+\left(\beta^2-\alpha\gamma\right)X^2, α,β,γ∈R\alpha,\beta,\gamma\in\mathbb R. If Y−1=[15−2515015−250015]Y^{-1}=\begin{bmatrix}\frac15&-\frac25&\frac15\\0&\frac15&-\frac25\\0&0&\frac15\end{bmatrix}, then (α−β+γ)2(\alpha-\beta+\gamma)^2 is equal to ______.

Answer and worked solution

Official answer: 100

Nilpotent X (X3=0X^3=0). The inverse of αI+βX+γX2\alpha I+\beta X+\gamma X^2 is 1αI−βα2X+(β2α3−γα2)X2\dfrac1\alpha I-\dfrac\beta{\alpha^2}X+\left(\dfrac{\beta^2}{\alpha^3}-\dfrac\gamma{\alpha^2}\right)X^2.

Compare with 15(I−2X+X2)\frac15(I-2X+X^2): α=5\alpha=5, β=10\beta=10, then 100125−γ25=15\frac{100}{125}-\frac\gamma{25}=\frac15 gives γ=15\gamma=15.

Value. (5−10+15)2=100(5-10+15)^2=100.

Answer: 100