KCET · Maths · Vector Algebra
If \(\overrightarrow{\mathbf{a}}\) and \(\overrightarrow{\mathbf{b}}\) are unit vectors and \(|\overrightarrow{\mathbf{a}}+\overrightarrow{\mathbf{b}}|=1\), then \(|\overrightarrow{\mathbf{a}}-\overrightarrow{\mathbf{b}}|\) is equal to
- A \(\sqrt{2}\)
- B 1
- C \(\sqrt{5}\)
- D \(\sqrt{3}\)
Answer & Solution
Correct Answer
(D) \(\sqrt{3}\)
Step-by-step Solution
Detailed explanation
Given, \(\quad|\overrightarrow{\mathbf{a}}+\overrightarrow{\mathbf{b}}|=1\)
\(\therefore \quad|\overrightarrow{\mathbf{a}}+\overrightarrow{\mathbf{b}}|^{2}=1^{2}\)
\(\Rightarrow|\overrightarrow{\mathbf{a}}|^{2}+|\overrightarrow{\mathbf{b}}|^{2}+2|\overrightarrow{\mathbf{a}}||\overrightarrow{\mathbf{b}}|=1\)
\(\Rightarrow \quad 2|\overrightarrow{\mathbf{a}}||\overrightarrow{\mathbf{b}}|=1-(1+1)\)
\(\Rightarrow \quad 2|\overrightarrow{\mathbf{a}}||\overrightarrow{\mathbf{b}}|=-1\)
Now, \(|\overrightarrow{\mathbf{a}}-\overrightarrow{\mathbf{b}}|^{2}=|\overrightarrow{\mathbf{a}}|^{2}+|\overrightarrow{\mathbf{b}}|^{2}-2|\overrightarrow{\mathbf{a}}||\overrightarrow{\mathbf{b}}|\) \(=1^{2}+1^{2}-(-1)=3\) [from Eq. (i)] \(\Rightarrow \quad|\overrightarrow{\mathbf{a}}-\overrightarrow{\mathbf{b}}|=\sqrt{3}\)
\(\therefore \quad|\overrightarrow{\mathbf{a}}+\overrightarrow{\mathbf{b}}|^{2}=1^{2}\)
\(\Rightarrow|\overrightarrow{\mathbf{a}}|^{2}+|\overrightarrow{\mathbf{b}}|^{2}+2|\overrightarrow{\mathbf{a}}||\overrightarrow{\mathbf{b}}|=1\)
\(\Rightarrow \quad 2|\overrightarrow{\mathbf{a}}||\overrightarrow{\mathbf{b}}|=1-(1+1)\)
\(\Rightarrow \quad 2|\overrightarrow{\mathbf{a}}||\overrightarrow{\mathbf{b}}|=-1\)
Now, \(|\overrightarrow{\mathbf{a}}-\overrightarrow{\mathbf{b}}|^{2}=|\overrightarrow{\mathbf{a}}|^{2}+|\overrightarrow{\mathbf{b}}|^{2}-2|\overrightarrow{\mathbf{a}}||\overrightarrow{\mathbf{b}}|\) \(=1^{2}+1^{2}-(-1)=3\) [from Eq. (i)] \(\Rightarrow \quad|\overrightarrow{\mathbf{a}}-\overrightarrow{\mathbf{b}}|=\sqrt{3}\)
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