Calculus 3 : Dot Product

Study concepts, example questions & explanations for Calculus 3

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Example Questions

Example Question #111 : Dot Product

Solve:

\(\displaystyle \left \langle 3\cos^2(x), 5e^y\right \rangle \cdot \left \langle 4\sin^2(x), 1\right \rangle\)

Possible Answers:

\(\displaystyle \left \langle12\cos^2(x)\sin^2(x), 5e^y \right \rangle\)

\(\displaystyle 12+5e^y\)

\(\displaystyle 12\cos^2(x)\sin^2(x)+e^y\)

\(\displaystyle 12\cos^2(x)\sin^2(x)+5e^y\)

Correct answer:

\(\displaystyle 12\cos^2(x)\sin^2(x)+5e^y\)

Explanation:

The dot product of two vectors is given by the sum of the products of the corresponding components (for example, \(\displaystyle \left \langle x, y\right \rangle \cdot \left \langle a,b\right \rangle= xa+yb\))

Our answer is

\(\displaystyle 12\cos^2(x)\sin^2(x)+5e^y\)

Example Question #427 : Vectors And Vector Operations

Find the dot product.

\(\displaystyle u=< 2,7>\)

\(\displaystyle v=< 5,10>\)

Possible Answers:

\(\displaystyle 80\)

\(\displaystyle 240\)

\(\displaystyle 36\)

\(\displaystyle 700\)

Correct answer:

\(\displaystyle 80\)

Explanation:

The dot product for the vectors

\(\displaystyle a=< a_1,a_2>\)

\(\displaystyle b=< b_1,b_2>\)

is defined as

\(\displaystyle a\bullet b=a_1*b_1+a_2*b_2\)

For the vectors in this problem we find that

\(\displaystyle u\bullet v=2(5)+7(10)=10+70=80\)

Example Question #428 : Vectors And Vector Operations

Find the dot  product.

\(\displaystyle u=< 1,0>\)

\(\displaystyle v=< 3,2>\)

Possible Answers:

\(\displaystyle 10\)

\(\displaystyle 3\)

\(\displaystyle 0\)

\(\displaystyle 6\)

Correct answer:

\(\displaystyle 3\)

Explanation:

The dot product for the vectors

\(\displaystyle a=< a_1,a_2>\)

\(\displaystyle b=< b_1,b_2>\)

is defined as

\(\displaystyle a\bullet b=a_1*b_1+a_2*b_2\)

For the vectors in this problem we find that

\(\displaystyle u\bullet v=1(3)+0(2)=3+0=3\)

Example Question #112 : Dot Product

Compute \(\displaystyle a\cdot b\), where \(\displaystyle a=\left \langle 3,0,4\right \rangle\) and \(\displaystyle b=\left \langle 9,11,2\right \rangle\).

Possible Answers:

\(\displaystyle \left \langle 2,11,2\right \rangle\)

\(\displaystyle 35\)

\(\displaystyle \left \langle -6,-11,4\right \rangle\)

\(\displaystyle 27\)

Correct answer:

\(\displaystyle 35\)

Explanation:

The formula for the dot product is 

\(\displaystyle a\cdot b=(i_1*i_2)+(j_1*j_2)+(k_1*k_2)\).

Using the given vectors, we get 

\(\displaystyle (3*9)+(0*11)+(4*2)=35\).

Example Question #113 : Dot Product

Compute the dot product of the vectors \(\displaystyle \left \langle 2,9,21\right \rangle\) and \(\displaystyle \left \langle 2,5,0\right \rangle\).

Possible Answers:

\(\displaystyle 3\)

\(\displaystyle 7\)

\(\displaystyle 49\)

\(\displaystyle \left \langle 4,45,0\right \rangle\)

Correct answer:

\(\displaystyle 49\)

Explanation:

The formula for the dot product of vectors 

\(\displaystyle a=\left \langle x_1,y_1,z_1\right \rangle\) 

and 

\(\displaystyle b=\left \langle x_2,y_2,z_2\right \rangle\) 

is 

\(\displaystyle a\cdot b=(x_1*x_2)+(y_1*y_2)+(z_1*z_2)\).

Using the vectors we were given, this becomes 

\(\displaystyle (2*2)+(9*5)+(21*0)\) which equals \(\displaystyle 49\)

Example Question #116 : Dot Product

Let a = (1,2,1), b = (2,3,2), and c = (2,0,4). Then \(\displaystyle (b\cdot c)a=\)

Possible Answers:

Cannot be determined

\(\displaystyle (12,0,24)\)

\(\displaystyle 4\sqrt{6}\)

\(\displaystyle (-4,8,-4)\)

\(\displaystyle 4\)

Correct answer:

\(\displaystyle (-4,8,-4)\)

Explanation:

Lets start by \(\displaystyle \vec{b}\cdot \vec{c}\):

\(\displaystyle \vec{b}\cdot \vec{c}=4+0-8=-4\)

Then we multiply \(\displaystyle -4\vec{a}\):

\(\displaystyle -4\vec{a}=(-4,8,-4)\)

Example Question #117 : Dot Product

Find all numbers x for which 2i+5j+2xk ⊥ 6i+4jxk:

Possible Answers:

\(\displaystyle x=4\)

\(\displaystyle x=\pm4\)

\(\displaystyle x=0,4\)

\(\displaystyle x=\pm1\)

\(\displaystyle x=1\)

Correct answer:

\(\displaystyle x=\pm4\)

Explanation:

if 2i+5j+2x⊥ 6i+4jxk, then the dot product of the two vectors should be 0.

\(\displaystyle (2,5,2x)\cdot (6,4,-x)=0\)

\(\displaystyle 12+20-2x^2=0\)

\(\displaystyle x^2=16\)

Therefore,

\(\displaystyle x=\pm4\) 

Example Question #118 : Dot Product

Find the dot product between the two vectors

\(\displaystyle u=< 5\sqrt3, 5>\)

\(\displaystyle v=< -2,2\sqrt3>\)

Possible Answers:

\(\displaystyle 225\)

\(\displaystyle 0\)

\(\displaystyle -300\)

\(\displaystyle 50\sqrt3\)

Correct answer:

\(\displaystyle 0\)

Explanation:

The dot product for the vectors

\(\displaystyle a=< a_1,a_2>\)

\(\displaystyle b=< b_1,b_2>\)

is defined as

\(\displaystyle a\bullet b=a_1*b_1+a_2*b_2\)

For the vectors in this problem we find that

\(\displaystyle u\bullet v=5\sqrt3(-2)+5(2\sqrt3)=-10\sqrt3+10\sqrt3=0\)

Example Question #119 : Dot Product

Find the dot product between \(\displaystyle \left \langle 3,-6,1\right \rangle\) and \(\displaystyle \left \langle 1,4,5\right \rangle\)

Possible Answers:

\(\displaystyle 8\)

\(\displaystyle 16\)

\(\displaystyle -16\)

\(\displaystyle \left \langle 3,-24,5\right \rangle\)

Correct answer:

\(\displaystyle -16\)

Explanation:

The formula for the dot product between two vectors \(\displaystyle a=\left \langle x_1,y_1,z_1\right \rangle\) and \(\displaystyle b=\left \langle x_2,y_2,z_2\right \rangle\) is \(\displaystyle a\cdot b=(x_1*x_2)+(y_1*y_2)+(z_1*z_2)\). Using the vectors in the problem statement, this becomes \(\displaystyle (3*1)+(-6*1)+(1*5)=-16\).

Example Question #112 : Dot Product

Evaluate the dot product \(\displaystyle u\bullet v\).

\(\displaystyle u=< a,b>\)

\(\displaystyle v=< 1,0>\)

Possible Answers:

\(\displaystyle a+b\)

\(\displaystyle b\)

\(\displaystyle 0\)

\(\displaystyle a\)

Correct answer:

\(\displaystyle a\)

Explanation:

The dot product for the vectors

\(\displaystyle r=< a_1,a_2>\)

\(\displaystyle s=< b_1,b_2>\)

is defined as

\(\displaystyle r\bullet s=a_1*b_1+a_2*b_2\)

For the vectors in this problem we find that

\(\displaystyle r\bullet s=a(1)+b(0)=a\)

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