Verified Solution • 2025–26 Session moderate

QJEEM23Ma0e10: JEE Main 2023 Physics - Class 11 JEEM23Ma0e10 Solution: Let and . If is a vector such that and , then is equal to…

Let a⃗=3i^+j^−k^\vec{a}=3 \hat{i}+\hat{j}-\hat{k} and c⃗=2i^−3j^+3k^\vec{c}=2 \hat{i}-3 \hat{j}+3 \hat{k} . If b⃗\vec{b} is a vector such that a⃗=b⃗×c⃗\vec{a}=\vec{b} \times \vec{c} and ∣b⃗∣2=50|\vec{b}|^{2}=50 , then ∣72−∣b⃗+c⃗∣2∣|72-| \vec{b}+\left.\vec{c}\right|^{2} \mid is equal to __________.

Step-by-Step Verified Solution

Given that a⃗=b⃗×c⃗\vec{a} = \vec{b} \times \vec{c} , we can find the magnitudes of a⃗\vec{a} and c⃗\vec{c} :

∣a⃗∣=32+12+(−1)2=11|\vec{a}| = \sqrt{3^2 + 1^2 + (-1)^2} = \sqrt{11}

∣c⃗∣=22+(−3)2+32=22|\vec{c}| = \sqrt{2^2 + (-3)^2 + 3^2} = \sqrt{22}

We know that the magnitude of the cross product of two vectors is equal to the product of the magnitudes of the vectors and the sine of the angle between them :

∣a⃗∣=∣b⃗×c⃗∣=∣b⃗∣∣c⃗∣sin⁡θ|\vec{a}| = |\vec{b} \times \vec{c}| = |\vec{b}||\vec{c}|\sin\theta

Plugging in the known values :

11=5022sin⁡θ\sqrt{11} = \sqrt{50}\sqrt{22}\sin\theta

Solving for the sine of the angle between the vectors :

sin⁡θ=110\sin\theta = \frac{1}{10}

Now we can find ∣b⃗+c⃗∣2|\vec{b} + \vec{c}|^2 using the formula :

∣b⃗+c⃗∣2=∣b⃗∣2+∣c⃗∣2+2b⃗⋅c⃗|\vec{b} + \vec{c}|^2 = |\vec{b}|^2 + |\vec{c}|^2 + 2\vec{b} \cdot \vec{c}

We have the dot product b⃗⋅c⃗=∣b⃗∣∣c⃗∣cos⁡θ\vec{b} \cdot \vec{c} = |\vec{b}||\vec{c}|\cos\theta , and we can use the relationship between sine and cosine: cos⁡θ=1−sin⁡2θ=9910\cos\theta = \sqrt{1 - \sin^2\theta} = \frac{\sqrt{99}}{10} .

Substitute the values into the formula :

∣b⃗+c⃗∣2=50+22+250229910=72+66|\vec{b} + \vec{c}|^2 = 50 + 22 + 2\sqrt{50}\sqrt{22}\frac{\sqrt{99}}{10} = 72 + 66

Finally, we need to find the absolute value of the difference :

∣72−∣b⃗+c⃗∣2∣=∣72−(72+66)∣=66|72 - |\vec{b} + \vec{c}|^2| = |72 - (72 + 66)| = 66

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