Q31 / 54 · JEE Advanced 2022 (Paper 2) · Mathematics

Let i^,j^ and k^ be the unit vectors along the three positive coordinate axes. Let

a→=3i^+j^-k^

b→=i^+b2j^+b3k^, b2,b3∈ℝ

c→=c1i^+c2j^+c3k^,c1,c2,c3∈ℝ

be three vectors such that b2b3>0,a→·b→=0 and 0-c3c2c30-c1-c2c101b2b3=3-c11-c2-1-c3. Then, which of the following is/are TRUE?

  • A
    a→·c→=0
  • Correct answer
    B
    b→·c→-=0
  • Correct answer
    C
    b→>10
  • Correct answer
    D
    c→≤11

Solution

Given,

a→=3i^+j^-k^

b→=i^+b2j^+b3k^

c→=c1i^+c2j^+c3k^

0-c3c2c30-c1-c2c101b2b3=3-c11-c2-1-c3

Now multiply and compare we get,

b2c3-b3c2=c1-3 .......1

c3-b3c1=1-c2...........2

c2-b2c1=1+c3...........3

Now applying operation equation 1i^-2j^+3k^ we get,

i^b2c3-c2b3-j^c3-b3c1+k^c2-b2c1=c1i^+c2j^+c3k^-3i^-j^+k^

⇒b→×c→=c→-a→      ....i

⇒b→×c→·b→=c→·b→-a→·b→

⇒b→·c→=0 given a→.b→=0

Again from (i)

c→×b→·c→=a→·c→-c→2=0

⇒c→2=a→c→cosθ, where θ=a→∧c→

⇒c→≥a→⇒c→≥11

Given that a→·b→=0⇒b2-b3+3=0

⇒b3-b2=3

Also b2-b3>0

Now b→=1+b22+b32

=1+b3-b22+2b2b3

=10+2b2b3

⇒b→2>10⇒b→>10

Hence, the correct answer is options: 'B', 'C', 'D'.

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