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Q. Let a=αˆi+ˆjˆk and b=2ˆi+ˆjαˆk,α>0 . If the projection of a×b on the vector ˆi+2ˆj2ˆk is 30 , then α is equal to

JEE MainJEE Main 2022Vector Algebra

Solution:

a×b=(1α)ˆi+(α22)ˆj+(α2)ˆk
Projection of a×b on ˆi+2ˆj2ˆk
=(a×b)(ˆi+2ˆj2ˆk)3=30
2α2α91=0
α=7,132