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Q.

Let a→=i^+2j^-3k^ and b→=2j^-3j^+5k^ . If r→×a→=b→×r→,r→·(αi^+2j^+k^)=3 and r→.(2i^+5j^-αk^)=-1,α∈R, Then the value of α+|r→|2 is equal to :

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a

11

b

9

c

15

d

13

answer is C.

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Detailed Solution

Given r¯×a¯=b¯×r¯it implies r¯×a¯+b¯=0¯hence, r¯=ka¯+b¯ =ki+2j−3k+2i−3j+5k=k3i−j+2k Given r¯⋅αi+2j+k=3⇒k3α−2+2=3⇒kα=1      ......1 Given  r¯⋅2i+5j−αk=−1⇒k6−5−2α=−1⇒k1−2α=−1.       ......2  From equations 1 and 2,α=1,k=1r=9+1+4=14α+r→2=1+14=15
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