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 #1
avatar+118651 
+1

A sine function has the following key features:

Period = π

Amplitude = 2

Midline: y=−2

y-intercept: (0, -2)

The function is a reflection of its parent function over the x-axis.

Use the sine tool to graph the function. The first point must be on the midline and the second point must be a maximum or minimum value on the graph closest to the first point.

 

the parent funtion is     y=sinx

this has a period of 2pi, and amplitude of 1 and a midline of y=0

 

\(y=asin[n(x+c)]+m\\ \text{a is the amplitude}\\ \text{m is the midline.}\\ \text{c is the phase shift that you probably don't need to worry about, it is zero}\\ \text{the period is }\frac{2\pi}{n} \)

 

Since it is reflected over the x axis, the a value will be negative so

 

Period = π                           n=2

Amplitude = 2                     a= - 2    (becasue of the xaxis reflection)

Midline: y=−2                      m=-2

y-intercept: (0, -2)               hopfully this one will just work.

 

 

Can you use that formula I just gave you to work out the equation of your sine curve?

Then graph it in desmos and check that it works :)

https://www.desmos.com/calculator

 

This is a really poor question  - they should not have been so many 2s!

 

anyway.....

 

Here is the graph.

Play with the cirlce on the left side.

Blank them all out then click on them one at a time from top to bottom and see how the parent graph is transformed. :)

https://www.desmos.com/calculator/jaujclijjn

Feb 5, 2019
 #2
avatar+26387 
+9

 If $\bold{a}$ and $\bold{b}$ are vectors such that $\|\bold{a}\| = 4$, $\|\bold{b}\| = 5$, and
 $\|\bold{a} + \bold{b}\| = 7$, then find $\|2a-3b\|$

 

If a and b are vectors such that \(||a||=4\), \(||b||=5\), and \(||a+b||=7\), then find \(||2a-3b||\).

 

1.) trigonometric
\(x = ||2\vec{a}-3\vec{b}||\)

 

\(\begin{array}{|rcll|} \hline 7^2 &=& 4^2+5^2- 2*4*5*\cos(A) \\ 49 &=& 16+25- 40\cos(A) \\ 40\cos(A) &=& 16+25-49 \\ 40\cos(A) &=& -8 \\ \mathbf{\cos(A)} &\mathbf{=}& \mathbf{-\dfrac{1}{5}} \\\\ x^2 &=&(2\cdot 4)^2+(3\cdot 5)^2-2*8*15*\cos(180^{\circ}-A )\\ x^2& =&8^2+15^2-16*15*\cos(180^{\circ}-A )\\ x^2&=& 64+225 + 240*\cos(A) \\ x^2&=& 289 + 240*\left(-\dfrac{1}{5}\right) \\ x^2&=& 289 - 48 \\ x^2&=& 241 \\ \mathbf{x }&\mathbf{=}& \mathbf{\sqrt{241 }} \\ \hline \end{array} \)

 

2.) vectorial

\(\text{Let $\vec{a}=\dbinom{x_a}{y_a} $ } \\ \text{Let $\vec{b}=\dbinom{x_b}{y_b} $ } \\ \text{Let $\vec{a}\cdot \vec{b}= x_a\cdot x_b+y_a\cdot y_b $ } \\ \text{Let $\vec{a}+\vec{b}=\dbinom{x_a+x_b}{y_a+y_b} $ } \\ \text{Let $2\vec{a}-3\vec{b}=\dbinom{2x_a-3x_b}{2y_a-3y_b} $ } \)

 

\(\begin{array}{|rcll|} \hline ||\vec{a}+\vec{b}|| &=& 7 \\ ||\vec{a}+\vec{b}||^2 &=& 7^2 \\ ||\vec{a}+\vec{b}||^2 &=& 49 \quad | \quad ||a+b||^2 = (x_a+x_b)^2 + (y_a+y_b)^2 \\ (x_a+x_b)^2 + (y_a+y_b)^2 &=& 49 \\ x_a^2 +2x_ax_b+x_b^2 + y_a^2 + 2y_ay_b+y_b^2 &=& 49 \\ x_a^2+y_a^2+x_b^2+y_b^2 +2(x_ax_b+ 2y_ay_b) &=& 49 \quad | \quad x_a^2+y_a^2 = ||\vec{a}||^2=4^2=16 \\ 16+x_b^2+y_b^2 +2(x_ax_b+ 2y_ay_b) &=& 49 \quad | \quad x_b^2+y_b^2 = ||\vec{b}||^2=5^2=25 \\ 16+25 +2(x_ax_b+ 2y_ay_b) &=& 49 \quad | \quad x_a\cdot x_b+y_a\cdot y_b = \vec{a}\cdot \vec{b}\\ 41 +2\vec{a}\cdot \vec{b} &=& 49 \\ 2\vec{a}\cdot \vec{b} &=& 8 \\ \mathbf{\vec{a}\cdot \vec{b}} &\mathbf{=}& \mathbf{4} \\ \hline \end{array}\)

 

\(\begin{array}{|rcll|} \hline ||2\vec{a}-3\vec{b}|| &=& ||\dbinom{2x_a-3x_b}{2y_a-3y_b} || \\ ||2\vec{a}-3\vec{b}||^2 &=& (2x_a-3x_b)^2 + (2y_a-3y_b)^2 \\ &=& 4x_a^2 -12x_ax_b + 9 x_b^2 + 4y_a^2 - 12y_ay_b + 9 y_b^2 \\ &=& 4(x_a^2+ y_a^2) + 9( x_b^2+y_b^2) -12(x_ax_b + y_ay_b) \\\\ && x_a^2+y_a^2 = ||\vec{a}||^2=4^2=16 \\ && x_b^2+y_b^2 = ||\vec{b}||^2=5^2=25 \\ && x_a\cdot x_b+y_a\cdot y_b = \vec{a}\cdot \vec{b}\\ \\ &=& 4\cdot 16 + 9\cdot 25 -12\vec{a}\cdot \vec{b} \quad | \quad \vec{a}\cdot \vec{b} =4\\ &=& 64 + 225 -12\cdot 4 \\ &=& 289 - 48\\ &=& 241 \\ \mathbf{||2\vec{a}-3\vec{b}||} &\mathbf{=}& \mathbf{\sqrt{241 }} \\ \hline \end{array}\)

 

laugh

Feb 5, 2019

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