Linear Algebra : Linear Algebra

Study concepts, example questions & explanations for Linear Algebra

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Example Questions

Example Question #1 : Norms

Find the norm of the vector \(\displaystyle \vec{v}= (4, 2, -5 )\)

Possible Answers:

\(\displaystyle 5 \sqrt{2}\)

\(\displaystyle 3 \sqrt{5}\)

\(\displaystyle 5\)

\(\displaystyle 9\)

\(\displaystyle 3\)

Correct answer:

\(\displaystyle 3 \sqrt{5}\)

Explanation:

\(\displaystyle || \vec{v} || = \sqrt{ 4^2 + 2^2 + (-5)^2 } = \sqrt{16 + 4 + 25 } = \sqrt{45}\)

This can be simplified:

\(\displaystyle \sqrt{3 \cdot 3 \cdot 5 } = 3 \sqrt {5}\)

Example Question #1 : Norms

Find the norm of the vector \(\displaystyle \vec{v} = (2, -3, -6 )\)

Possible Answers:

\(\displaystyle \sqrt{23}\)

\(\displaystyle 6\)

\(\displaystyle 7\)

\(\displaystyle \sqrt{41 }\)

Correct answer:

\(\displaystyle 7\)

Explanation:

\(\displaystyle || \vec{v} || = \sqrt{ 2^2 + (-3)^2 + (-6)^2 } = \sqrt{4 + 9 + 36 } = \sqrt{49} = 7\)

Example Question #221 : Linear Algebra

Find the norm of the vector \(\displaystyle \vec{v} = (3, -2, -2)\)

Possible Answers:

\(\displaystyle 1\)

\(\displaystyle \sqrt{17}\)

\(\displaystyle \sqrt{15}\)

\(\displaystyle \sqrt{7}\)

Correct answer:

\(\displaystyle \sqrt{17}\)

Explanation:

\(\displaystyle || \vec{v} || = \sqrt{3^2 + (-2)^2 + (-2)^2 } = \sqrt{9+4+4} = \sqrt{17}\)

Example Question #222 : Linear Algebra

Find the norm of the vector \(\displaystyle \vec{v} = (4, -1, 3 )\)

Possible Answers:

\(\displaystyle \sqrt{26}\)

\(\displaystyle \sqrt{24}\)

\(\displaystyle \sqrt{6}\)

\(\displaystyle 5\)

Correct answer:

\(\displaystyle \sqrt{26}\)

Explanation:

\(\displaystyle || \vec{v} || = \sqrt{ 4^2 + (-1)^2 + 3^2 } = \sqrt{16+1+9 } = \sqrt{26}\)

Example Question #223 : Linear Algebra

Find the norm of the vector \(\displaystyle \vec{v} = (3, 4, 5 )\)

Possible Answers:

\(\displaystyle 5 \sqrt{2}\)

\(\displaystyle 3 \sqrt{2}\)

\(\displaystyle 2\sqrt{5}\)

\(\displaystyle 2 \sqrt{3}\)

Correct answer:

\(\displaystyle 5 \sqrt{2}\)

Explanation:

\(\displaystyle || \vec{v} || = \sqrt{ 3^2 + 4^2 + 5^2 } = \sqrt{9+16+25} = \sqrt{50 }\)

This can be simplified:

\(\displaystyle \sqrt{2 \cdot 5 \cdot 5 } = 5 \sqrt{2 }\)

Example Question #144 : Operations And Properties

Let \(\displaystyle \textbf{u} = (2, 3, a, 1 )\) for some real number \(\displaystyle a\).

Give \(\displaystyle a\) such that \(\displaystyle \left \| \textbf{u} \right \| = 4\).

Possible Answers:

\(\displaystyle -2\)

\(\displaystyle \pm \sqrt{2}\)

\(\displaystyle \pm \frac{16}{13}\)

\(\displaystyle \frac{2}{3}\)

\(\displaystyle \pm \frac{4 \sqrt{13}}{13}\)

Correct answer:

\(\displaystyle \pm \sqrt{2}\)

Explanation:

\(\displaystyle \left \| \textbf{u} \right \|\), the norm, or length, of vector \(\displaystyle \textbf{u}\), is equal to the square root of the sum of the squares of its elements. Therefore, 

\(\displaystyle \left \| \textbf{u} \right \| = \sqrt{2^{2}+3^{2}+a^{2}+1^{2}}\)

\(\displaystyle = \sqrt{4+9+a^{2}+1}\)

\(\displaystyle = \sqrt{ a^{2}+14}\)

Set this equal to 4:

\(\displaystyle \sqrt{ a^{2}+14} = 4\)

\(\displaystyle a^{2}+14 = 16\)

\(\displaystyle a^{2}= 2\)

\(\displaystyle a = \pm \sqrt{2}\)

Example Question #145 : Operations And Properties

\(\displaystyle \textbf{u}= (-3, 2, x, x)\),

where \(\displaystyle x\) is a real number.

In terms of \(\displaystyle x\), give \(\displaystyle \left \| \textbf{u} \right \|\).

Possible Answers:

\(\displaystyle \sqrt{2x-1}\)

\(\displaystyle x \sqrt{6}\)

\(\displaystyle \sqrt{2 x ^{2} +13}\)

\(\displaystyle \sqrt{2x+5}\)

\(\displaystyle \sqrt{2 x ^{2} -5}\)

Correct answer:

\(\displaystyle \sqrt{2 x ^{2} +13}\)

Explanation:

\(\displaystyle \left \| \textbf{u} \right \|\), the norm, or length, of vector \(\displaystyle \textbf{u}\), is equal to the square root of the sum of the squares of its elements. Therefore, 

\(\displaystyle \left \| \textbf{u} \right \|= \sqrt{(-3)^{2}+ 2 ^{2}+ x ^{2} +x ^{2}}\)

\(\displaystyle =\sqrt{9+4+ x ^{2} +x ^{2}}\)

\(\displaystyle =\sqrt{2 x ^{2} +13}\)

Example Question #146 : Operations And Properties

\(\displaystyle \bold{v} = \left ( \frac{1}{3}, - \frac{1}{3},\frac{1}{3} \right )\) 

True or false: \(\displaystyle \textbf{v}\) is an example of a unit vector.

Possible Answers:

True

False

Correct answer:

False

Explanation:

\(\displaystyle \textbf{v}\) is a unit vector if and only if its norm, or length, \(\displaystyle \left \| \textbf{v} \right \|\) - the square root of the sum of the squares of its elements - is equal to 1. Find the length using this definition: 

\(\displaystyle \left \| \textbf{v} \right \| = \sqrt{\left ( \frac{1}{3} \right )^{2}+ \left ( -\frac{1}{3} \right )^{2}+ \left ( \frac{1}{3} \right )^{2}}\)

\(\displaystyle = \sqrt{\frac{1}{9}+ \frac{1}{9} + \frac{1}{9} }\)

\(\displaystyle = \sqrt{\frac{1}{3} }\)

\(\displaystyle \left \| \textbf{v} \right \| \ne 1\), so \(\displaystyle \textbf{v}\) is not a unit vector.

Example Question #14 : Norms

\(\displaystyle \bold{v} = \left ( \frac{1}{2},\frac{ \sqrt{2}}{2}, -\frac{1}{2} \right )\) 

True or false: \(\displaystyle \textbf{v}\) is an example of a unit vector.

Possible Answers:

False

True

Correct answer:

True

Explanation:

\(\displaystyle \textbf{v}\) is a unit vector if and only if its norm, or length, \(\displaystyle \left \| \textbf{v} \right \|\) - the square root of the sum of the squares of its elements - is equal to 1. Find the length using this definition: 

 

\(\displaystyle \left \| \textbf{v} \right \| = \sqrt{\left ( \frac{1}{2} \right )^{2}+ \left ( \frac{\sqrt{2}}{2} \right ) ^{2}+ \left ( -\frac{1}{2} \right ) ^{2}}\)

\(\displaystyle =\sqrt{ \frac{1}{4} + \frac{2}{4} + \frac{1}{4} }\)

\(\displaystyle =\sqrt{1}\)

\(\displaystyle =1\)

\(\displaystyle \textbf{v}\) is a unit vector.

Example Question #15 : Norms

Which of these functions could be that of a Euclidean norm operator? You may assume each function is onto.

Possible Answers:

All of the other answers are norm operators

\(\displaystyle h: \mathbb{N}^3 \rightarrow \mathbb{Q}\)

\(\displaystyle f:\mathbb{R}^7 \rightarrow \mathbb{R}\)

\(\displaystyle g: \mathbb{Q}^2 \rightarrow \mathbb{N}\)

Correct answer:

\(\displaystyle f:\mathbb{R}^7 \rightarrow \mathbb{R}\)

Explanation:

This function's range is \(\displaystyle \mathbb{R}\), the set of all real numbers. In short, this is set of all possible "distances between two given numbers" in elementary linear algebra. \(\displaystyle h\) would not be a norm. For example, \(\displaystyle h(1,1,1) = \sqrt{1+1+1} =\sqrt{3}\), which is not a rational number (part of \(\displaystyle \mathbb{Q}\)). Similarly, \(\displaystyle g\) is also not a norm. We have\(\displaystyle g(1,1) = \sqrt{1+1} = \sqrt{2}\), which is not a natural number.

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