Let a→=2i^+j^-k^ and b→=i^+2j^+k^ be two vectors. Consider a vector c→=αa→+βb→, α, β∈R. If the projection of c→ on the vector a→+b→ is 32, then the minimum value of c→-a→×b→.c→ equals

JEE Advanced · Mathematics · 30. Vector Algebra

Let $\vec{a} = 2 \hat{i} + \hat{j} - \hat{k}$ and $\vec{b} = \hat{i} + 2 \hat{j} + \hat{k}$ be two vectors. Consider a vector $\vec{c} = α \vec{a} + β \vec{b}, α, β \in R .$ If the projection of $\vec{c}$ on the vector $(\vec{a} + \vec{b})$ is $3 \sqrt{2},$ then the minimum value of $(\vec{c} - (\vec{a} \times \vec{b})) . \vec{c}$ equals

A 15
B 20
C 25
D 18

Verified Solution

Answer & Solution

Correct Answer

(D) 18

Step-by-step Solution

Detailed explanation

\begin{matrix} ∵ \vec{a} + \vec{b} = 3 i + 3 j & ∵ \vec{a} = 2 i + j - k, & |\vec{a}| = \sqrt{6} \end{matrix}

\begin{matrix} \Rightarrow |\vec{a} + \vec{b}| = 3 \sqrt{2}, & \vec{b} = i + 2 j + k, & |\vec{b}| = \sqrt{6} \end{matrix}

\vec{a} . \vec{b} = 2 + 2 - 1 = 3

Now,
as given projection of

\vec{a} + \vec{b}

\vec{c} = 3 \sqrt{2}

\frac{(\vec{a} + \vec{b})}{|\vec{a} + \vec{b}|} . \vec{c} = 3 \sqrt{2}

(\vec{a} + \vec{b}) . (α \vec{a} + β \vec{b}) = 18

α {|\vec{a}|}^{2} + α (\vec{a} . \vec{b}) + β {|\vec{b}|}^{2} + β (\vec{a} . \vec{b}) = 18

as given

6 α + 3 α + 6 β + 3 β = 18

\begin{matrix} \vec{c} = α \vec{a} + β \vec{b}, & α + β = 2 \end{matrix}

…(i)
Since

\vec{a}, \vec{b}

and

\vec{c}

are co-planar hence

(\vec{a} \times \vec{b}) . \vec{c} = 0

Minimum value of

(\vec{c} - (\vec{a} \times \vec{b})) . \vec{c} =

Minimum of

\vec{c} . \vec{c}

=

Minimum of

{|\vec{c}|}^{2}

(∵ (\vec{a} \times \vec{b}) . \vec{c} = 0)

now

\vec{c} = α \vec{a} + β \vec{b}

and

α + β = 2

\vec{c} = α (2 i + j - k) + (2 - α) (i + 2 j + k) =

\vec{c} = (2 + α) i + (4 - α) j + k (2 - 2 α)

{|\vec{c}|}^{2} = 6 (α^{2} - 2 α + 4)

\begin{matrix} M i n i m u m {|\vec{c}|}^{2} = 18 & (∴ M i n i m u m o f a n^{2} + b n + c = - \frac{D}{4 a} = - \frac{b^{2} - 4 a c}{4 a}) \end{matrix}

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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