AP EAMCET · Maths · Vector Algebra

Consider the vectors \(u=a \hat{\mathbf{i}}+b \hat{\mathbf{j}}+c \hat{\mathbf{k}}\), \(v=a^2 \hat{\mathbf{i}}+b^2 \hat{\mathbf{j}}+c^2 \hat{\mathbf{k}}\) and \(w=a^3 \hat{\mathbf{i}}+b^3 \hat{\mathbf{j}}+c^3 \hat{\mathbf{k}}\).
These vectors are coplanar if and only if

A all \(a, b\) and \(c\) are equal
B one of \(a, b\) and \(c\) is zero
C any two of \(a, b\) and \(c\) are equal
D either one of \(a, b\) and \(c\) is zero, or any two of \(a, b\) and \(c\) are equal

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Answer & Solution

Correct Answer

(D) either one of \(a, b\) and \(c\) is zero, or any two of \(a, b\) and \(c\) are equal

Step-by-step Solution

Detailed explanation

Here \[ \begin{aligned} \mathbf{u} & =a \hat{\mathbf{i}}+b \hat{\mathbf{J}}+c \hat{\mathbf{k}} \\ \mathbf{v} & =a^2 \hat{\mathbf{i}}+b^2 \hat{\mathbf{j}}+c^2 \hat{\mathbf{k}} \\ \mathbf{w} & =a^3 \hat{\mathbf{i}}+b^3 \hat{\mathbf{j}}+c^3 \hat{\mathbf{k}} \end{aligned} \]…

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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Consider the vectors \(u=a \hat{\mathbf{i}}+b \hat{\mathbf{j}}+c \hat{\mathbf{k}}\), \(v=a^2 \hat{\mathbf{i}}+b^2 \hat{\mathbf{j}}+c^2 \hat{\mathbf{k}}\) and \(w=a^3 \hat{\mathbf{i}}+b^3 \hat{\mathbf{j}}+c^3 \hat{\mathbf{k}}\).These vectors are coplanar if and only if

Step-by-step Solution

Consider the vectors \(u=a \hat{\mathbf{i}}+b \hat{\mathbf{j}}+c \hat{\mathbf{k}}\), \(v=a^2 \hat{\mathbf{i}}+b^2 \hat{\mathbf{j}}+c^2 \hat{\mathbf{k}}\) and \(w=a^3 \hat{\mathbf{i}}+b^3 \hat{\mathbf{j}}+c^3 \hat{\mathbf{k}}\).
These vectors are coplanar if and only if