JEE MAIN - Mathematics (2025 - 22nd January Evening Shift - No. 2)
Explanation
$$ P = (-2, -1, 3) $$
Since the line through $ P $ is parallel to the vector
$$ \vec{d} = (3, 2, 2), $$
any point $ Q $ on this line can be expressed as:
$$ Q = P + t\,\vec{d} = (-2 + 3t,\, -1 + 2t,\, 3 + 2t), $$
where $ t $ is a real number. Given that $ P $ and $ Q $ must be distinct, we require $ t \neq 0 $.
The point $ R $ is given by:
$$ R = (1, 3, 3). $$
The distance from $ Q $ to $ R $ is $ 5 $, so we have:
$$ \left[ (-2 + 3t - 1)^2 + (-1 + 2t - 3)^2 + (3 + 2t - 3)^2 \right] = 5^2. $$
Simplify each coordinate difference:
For the $ x $-coordinate:
$$ -2 + 3t - 1 = 3t - 3 = 3(t - 1). $$
For the $ y $-coordinate:
$$ -1 + 2t - 3 = 2t - 4 = 2(t - 2). $$
For the $ z $-coordinate:
$$ 3 + 2t - 3 = 2t. $$
So the equation becomes:
$$ [3(t - 1)]^2 + [2(t - 2)]^2 + (2t)^2 = 25. $$
Expanding the squares:
$$ 9(t - 1)^2 + 4(t - 2)^2 + 4t^2 = 25. $$
Expand each term:
$$ 9(t^2 - 2t + 1) + 4(t^2 - 4t + 4) + 4t^2 = 25, $$
which simplifies to:
$$ 9t^2 - 18t + 9 + 4t^2 - 16t + 16 + 4t^2 = 25. $$
Combine like terms:
$$ (9t^2 + 4t^2 + 4t^2) - (18t + 16t) + (9 + 16) = 25, $$
$$ 17t^2 - 34t + 25 = 25. $$
Subtract 25 from both sides:
$$ 17t^2 - 34t = 0. $$
Factor out $ 17t $:
$$ 17t(t - 2) = 0. $$
Since $ t \neq 0 $, we must have:
$$ t = 2. $$
Substitute $ t = 2 $ back into the equation for $ Q $:
$$ Q = (-2 + 3(2),\, -1 + 2(2),\, 3 + 2(2)) = (4, 3, 7). $$
Next, to find the square of the area of triangle $ PQR $, first compute the vectors:
$$ \vec{PQ} = Q - P = (4 - (-2),\, 3 - (-1),\, 7 - 3) = (6, 4, 4), $$
$$ \vec{PR} = R - P = (1 - (-2),\, 3 - (-1),\, 3 - 3) = (3, 4, 0). $$
The area of the triangle is given by:
$$ \text{Area} = \frac{1}{2} \, \| \vec{PQ} \times \vec{PR} \|. $$
Calculate the cross product:
$$ \vec{PQ} \times \vec{PR} = \begin{vmatrix} \hat{i} & \hat{j} & \hat{k} \\ 6 & 4 & 4 \\ 3 & 4 & 0 \\ \end{vmatrix}. $$
Expanding the determinant:
$ \hat{i} $-component:
$$ 4 \times 0 - 4 \times 4 = -16, $$
$ \hat{j} $-component:
$$ -(6 \times 0 - 4 \times 3) = 12, $$
$ \hat{k} $-component:
$$ 6 \times 4 - 4 \times 3 = 24 - 12 = 12. $$
Thus,
$$ \vec{PQ} \times \vec{PR} = (-16,\, 12,\, 12). $$
Find the magnitude squared of the cross product:
$$ \| \vec{PQ} \times \vec{PR} \|^2 = (-16)^2 + 12^2 + 12^2 = 256 + 144 + 144 = 544. $$
The square of the area of triangle $ PQR $ is:
$$ (\text{Area})^2 = \left(\frac{1}{2}\right)^2 \| \vec{PQ} \times \vec{PR} \|^2 = \frac{1}{4} \times 544 = 136. $$
Thus, the square of the area of $ \triangle PQR $ is $$136.$$
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