Let a→=i^-j^,b→=i^+j^+k^ and c¯ be a vector suchthat a¯×c¯+b→=0→ and a→·c→=4 then |c→|2 is equal to

A
8
B
19/2
C
9
D
17/2

Solution:

${|\bar{a} \times \bar{c}|}^{2} + {(\bar{a} . \bar{c})}^{2} = {|\bar{a}|}^{2} {|\bar{c}|}^{2} \Rightarrow {\bar{|b|}}^{2} + 16 = 2 {|\bar{c}|}^{2} \Rightarrow 3 + 16 = 2 {|\bar{c}|}^{2} \Rightarrow {|\bar{c}|}^{2} = \frac{19}{2}$

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