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JEE Main 20264 AprilEvening Shift
Exam
JEE Main
Year
2026
Date
4 April
Shift
Evening Shift
Subject
Mathematics
Chapter
Matrices

JEE Main 2026 · 4 April · Evening Shift · Mathematics · Matrices

Let A= bmatrix 1 & 2 & 7 4 & -2 & 8 3 & 8 & -7 bmatrix and (A-α...

Let A=[1274−2838−7]A=\begin{bmatrix} 1 & 2 & 7 \\ 4 & -2 & 8 \\ 3 & 8 & -7 \end{bmatrix} and det⁡(A−αI)=0\det(A-\alpha I)=0, where α\alpha is a real number. If the largest possible value of α\alpha is pp, then the circle (x−p)2+(y−2p)2=320(x-p)^2+(y-2p)^2=320, intersects the co-ordinate axes at

Options

  1. A11 point
  2. B22 points
  3. C33 points
  4. D44 points

Correct answer

C. 33 points

Step-by-step solution

The characteristic equation is given by det⁡(A−αI)=0\det(A-\alpha I) = 0.

det⁡[1−α274−2−α838−7−α]=0\det \begin{bmatrix} 1-\alpha & 2 & 7 \\ 4 & -2-\alpha & 8 \\ 3 & 8 & -7-\alpha \end{bmatrix} = 0

Expanding the determinant along the first row:

(1−α)[(−2−α)(−7−α)−64]−2[4(−7−α)−24]+7[32−3(−2−α)]=0(1-\alpha)[(-2-\alpha)(-7-\alpha) - 64] - 2[4(-7-\alpha) - 24] + 7[32 - 3(-2-\alpha)] = 0

(1−α)(α2+9α−50)−2(−4α−52)+7(3α+38)=0(1-\alpha)(\alpha^2 + 9\alpha - 50) - 2(-4\alpha - 52) + 7(3\alpha + 38) = 0

−α3−8α2+59α−50+8α+104+21α+266=0-\alpha^3 - 8\alpha^2 + 59\alpha - 50 + 8\alpha + 104 + 21\alpha + 266 = 0

α3+8α2−88α−320=0\alpha^3 + 8\alpha^2 - 88\alpha - 320 = 0

By inspection, α=8\alpha = 8 is a root since 83+8(82)−88(8)−320=512+512−704−320=08^3 + 8(8^2) - 88(8) - 320 = 512 + 512 - 704 - 320 = 0.

Factoring out (α−8)(\alpha - 8), we get:

(α−8)(α2+16α+40)=0(\alpha - 8)(\alpha^2 + 16\alpha + 40) = 0

The roots are α=8\alpha = 8 and α=−16±256−1602=−8±26\alpha = \dfrac{-16 \pm \sqrt{256 - 160}}{2} = -8 \pm 2\sqrt{6}.

The largest possible value of α\alpha is p=8p = 8.

Substituting p=8p = 8 into the given equation of the circle:

(x−8)2+(y−16)2=320(x-8)^2 + (y-16)^2 = 320

To find the intersection points with the x-axis, substitute y=0y = 0:

(x−8)2+(−16)2=320(x-8)^2 + (-16)^2 = 320

(x−8)2+256=320⇒(x−8)2=64⇒x−8=±8(x-8)^2 + 256 = 320 \Rightarrow (x-8)^2 = 64 \Rightarrow x-8 = \pm 8

x=0x = 0 or x=16x = 16

The circle intersects the x-axis at (0,0)(0, 0) and (16,0)(16, 0).

To find the intersection points with the y-axis, substitute x=0x = 0:

(−8)2+(y−16)2=320(-8)^2 + (y-16)^2 = 320

64+(y−16)2=320⇒(y−16)2=256⇒y−16=±1664 + (y-16)^2 = 320 \Rightarrow (y-16)^2 = 256 \Rightarrow y-16 = \pm 16

y=0y = 0 or y=32y = 32

The circle intersects the y-axis at (0,0)(0, 0) and (0,32)(0, 32).

The distinct points of intersection with the coordinate axes are (0,0)(0, 0), (16,0)(16, 0), and (0,32)(0, 32).

Thus, the circle intersects the coordinate axes at exactly 33 points.

Answer: 33 points
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