TS EAMCET · Maths · Three Dimensional Geometry

Let $3 \hat{i} + \hat{j} - \hat{k}$ be the position vector of a point $B$ . Let $A$ be a point on the line which is passing through $B$ and parallel to the vector $2 \hat{i} - \hat{j} + \hat{k}$ . If $|\vec{B A}| = 18$ , then the position vector of $A$ is

A $- 9 \hat{i} + 7 \hat{j} - 13 \hat{k}$
B $- 9 \hat{i} + 3 \hat{j} + 12 \hat{k}$
C $9 \hat{i} - 3 \hat{j} + 2 \hat{k}$
D $3 \hat{i} - \hat{j} + 7 \hat{k}$

Verified Solution

Answer & Solution

Correct Answer

(A) $- 9 \hat{i} + 7 \hat{j} - 13 \hat{k}$

Step-by-step Solution

Detailed explanation

Vector equation of the given line is, r=3i^+j^-k^+t2i^-j^+2k^ ⇒BA→=t2i^-j^+2k^=2i^t-j^t+2k^t ⇒18=4t2+t2+4t2=±3t ∴ t=±6 ∴ OA→=3i^+j^-k^±62i^-j^+2k^ =3i^+12i^+j^-6j^-k^+12k^ =15i^-5j^+11k^ or =3i^-12i^+j^+6j^-k^-12k^…

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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Let 3i^+j^-k^ be the position vector of a point B. Let A be a point on the line which is passing through B and parallel to the vector 2i^-j^+k^. If BA→=18, then the position vector of A is

Step-by-step Solution

Let $3 \hat{i} + \hat{j} - \hat{k}$ be the position vector of a point $B$ . Let $A$ be a point on the line which is passing through $B$ and parallel to the vector $2 \hat{i} - \hat{j} + \hat{k}$ . If $|\vec{B A}| = 18$ , then the position vector of $A$ is