For any vector $\vec{a}=a_{1} \hat{i}+a_{2} \hat{j}+a_{3} \hat{k}$, with $10\left|a_{i}\right|<1, i=1,2,3$, consider the following statements :
(A): $$\max \left\{\left|a_{1}\right|,\left|a_{2}\right|,\left|a_{3}\right|\right\} \leq|\vec{a}|$$
(B) : $$|\vec{a}| \leq 3 \max \left\{\left|a_{1}\right|,\left|a_{2}\right|,\left|a_{3}\right|\right\}$$
Solution
We have,
<br/><br/>$$
\begin{aligned}
& 10\left|a_i\right|<1, i=1,2,3 \\\\
& \text { Let } \left|a_1\right| \geq\left|a_2\right| \geq\left|a_3\right| \\\\
& |\vec{a}|=\sqrt{a_1^2+a_2^2+a_3^2} \geq \sqrt{a_1^2} \\\\
& \therefore|\vec{a}| \geq\left|a_1\right| \text { or } \max \left\{\left|a_1\right|,\left|a_2\right|,\left|a_3\right|\right\} \text {. }
\end{aligned}
$$
<br/><br/>Hence, (A) is true.
<br/><br/>$$
\begin{array}{rlrl}
& |\vec{a}| =\sqrt{a_1^2+a_2^2+a_3^2} \leq \sqrt{a_1^2+a_1^2+a_1^2} \\\\
& =\sqrt{3}\left|a_1\right| \\\\
\therefore & |\vec{a}|=\sqrt{3}\left|a_1\right|<3\left|a_1\right| \\\\
\therefore & |\vec{a}|<3 \max \left\{\left|a_1\right|,\left|a_2\right|,\left|a_3\right|\right\}
\end{array}
$$
<br/><br/>Hence, (B) is also true.
About this question
Subject: Mathematics · Chapter: Vector Algebra · Topic: Types of Vectors
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