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Let a⃗=2i^+3j^−4k^\vec{a} = 2\hat{i} + 3\hat{j} - 4\hat{k} and b⃗=3i^−5j^+6k^\vec{b} = 3\hat{i} - 5\hat{j} + 6\hat{k}. Let c⃗\vec{c} be a vector such that c⃗×a⃗=b⃗×c⃗\vec{c} \times \vec{a} = \vec{b} \times \vec{c} and c⃗⋅(2a⃗−3b⃗)=2382\vec{c}\cdot(2\vec{a}-3\vec{b}) = 238\sqrt{2} then ∣c⃗∣2|\vec{c}|^2 is equal to :

Solution

✅ Correct Option: 3

c⃗×a⃗=b⃗×c⃗=−c⃗×b⃗\vec{c}\times\vec{a} = \vec{b}\times\vec{c} = -\vec{c}\times\vec{b}, so c⃗×(a⃗+b⃗)=0\vec{c}\times(\vec{a}+\vec{b}) = 0, meaning c⃗\vec{c} parallel to a⃗+b⃗=5i^−2j^+2k^\vec{a}+\vec{b} = 5\hat{i}-2\hat{j}+2\hat{k}. Let c⃗=t(a⃗+b⃗)\vec{c}=t(\vec{a}+\vec{b}). 2a⃗−3b⃗=−5i^+21j^−26k^2\vec{a}-3\vec{b} = -5\hat{i}+21\hat{j}-26\hat{k}. Dot: t[(−25)+(−42)+(−52)]=−119t=2382t[(-25)+(-42)+(-52)] = -119t = 238\sqrt{2}, so t=−22t=-2\sqrt{2}. ∣c⃗∣2=8⋅(25+4+4)=8⋅33=264|\vec{c}|^2 = 8 \cdot (25+4+4) = 8\cdot 33 = 264.

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