WBJEE · Maths · Vector Algebra

Which of the following is not always true?

A \(\mid a+b\mid^{2}=|a|^2+|b|^2. \text{if a and b are perpendicular to each other}\)
B \(\mid a+\lambda b| \geq|a|\) for all \(\lambda \in R\) if a and b are perpendicular to each other
C \(\mid a+b \mid^{2}+\mid a-b \mid^{2}=2(|a|^2+|b|^2)\)
D \(\mid a+\lambda b| \geq|a|\) for all \(\lambda \in R\) if a is parallel to b

Verified Solution

Answer & Solution

Correct Answer

(D) \(\mid a+\lambda b| \geq|a|\) for all \(\lambda \in R\) if a is parallel to b

Step-by-step Solution

Detailed explanation

(a) If a and b are perpendicular to each other, then \(\mathbf{a} \cdot \mathbf{b}=0\) Now consider, \(\begin{aligned} |\mathbf{a}+\mathbf{b}|^{2} &=(\mathbf{a}+\mathbf{b}) \cdot(\mathbf{a}+\mathbf{b}) \\ &=|\mathbf{a}|^{2}+|\mathbf{b}|^{2} \end{aligned}\) So, option (a) is…

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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