a=2i^+j^−2k^,b=i^+j^ ⇒∣a∣=3 and a×b=i^21j^11k^−20=2i^−2j^+k^ ∣a×b∣=4+4+1=3 Now, ∣c−a∣=22⇒∣c−a∣2=8 ⇒∣c−a∣⋅(c−a)=8 ⇒∣c∣2+∣a∣2−2c⋅a=8 ⇒∣c∣2+9−2∣c∣=8 ⇒(∣c∣−1)2=0⇒∣c∣=1 ∴∣(a×b)×c∣=∣a×b∣∣c∣sin30∘=3×1×21=23
JEE Main 2013 — Mathematics Vectors & 3D Geometry
Let a=2i^+j^−2k^,b=i^+j^. If c is a vector such that a∙c=∣c∣,∣c−a∣=22 and the angle between a×b and c is 30∘, then ∣(a×b)×c∣ equals:
Held on 25 Apr 2013 · Verified 6 Jul 2026.
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