SlopeIntercept Form
Description SlopeIntercept Form
To write linear equations, there are three forms: slopeintercept form, pointslope form and standard from. For this video, we will investigate the very popular slopeintercept form.
Equations written in this form follow this format: y = mx + b. The mvalue represents the slope which is the rise over the run of the line – the steepness of the line. The bvalue represents the yintercept, the point where the line touches the yaxis, and the xcoordinate is equal to zero. You can learn a lot from an equation of a line written in slopeintercept form.
Let’s look at an example: y = 2x + 5. From this linear equation, I can tell the line is sloping down because the mvalue is a negative number. The slope is somewhat steep because the larger the mvalue the more steep the line. I can also tell the line touches the yaxis at the ordered pair ( 0, 5).
That’s a lot of information from an equation with only three terms. No wonder teachers often tell their students to write equations of the line in slopeintercept form. To learn more about this form, and some practical uses in the real world, take a look at this video.
Use slope to create equations of the line. CCSS.MATH.CONTENT.HSG.GPE.B.5
Transcript SlopeIntercept Form
NASA is planning a special project for Valentine's DayMission4Love. They want to show a live sunset from Mars on national television.
In order to get the best view, they plan to send the Mars Rover to the top of a mountain. To complete the mission successfully, NASA has to figure out if the Rover has enough power to reach the top of the mountain and which route he has to take. We can help NASA by using slopeintercept form.
Introduction to SlopeIntercept Form & Formula
There are three common forms to write linear equations:
 SlopeIntercept form
 PointSlope form
 Standard form
Today we will focus on SlopeIntercept form. As you can see, the slope is explicitly included in this form. Let's recall what slope is:
 Slope shows the steepness of a line.
 It can be calculated by "rise over run".
 If you take any two points on the line, you can calculate the slope using this formula: m equals the change of the two ycoordinates divided by the change of the two xcoordinates, or delta y over delta x for short. The Greek letter Delta means 'change' in math expressions.
So, what about about the intercepts? Let's look at the formula for the SlopeIntercept form. Remember, M represents the slope of the line, while b is the yintercept.
YIntercept
Lets look at the graph of y=2x+1 in the coordinate plane.
The yintercept is the point where the line crosses the yaxis. Here, the line crosses the yaxis at positive 1.
What happens if you change the yintercept to 2? As you can see, the slope stays the same, but the whole line is shifted up one unit.
What happens if you change the yintercept to 1? As you can see, the slope stays the same, but the whole line is shifted down by two units.
Slope
Now that we understand the concept of the yintercept, let's look at slope.
Here, the slope is positive 2, meaning the graph is increasing from left to right. For every unit you go to the right, you must go up 2 units.
If we let m equal 1, the yintercept stays the same, but now the slope is different. It still increases from left to right, but for every 1 unit you go to the right, you must also go up 1 unit.
If you change the slope to be zero, the yintercept stays the same, but the line becomes horizontal. It is neither increasing nor decreasing.
Let's try changing the slope to −1. Again, the yintercept stays the same, but the line is now decreasing from left to right because it has a negative slope. Now, for every unit right, you must go down one unit. Now that you've gotten the hang of slopeintercept form, let's get back to our Mars Rover.
SlopeIntercept Form Example 1
The Rover currently has 20 units of solar battery power left. If it uses 2 units of battery power per mile, how many miles can the Rover drive before it runs out of battery? Can it reach a spot that is 8 miles away?
Let's put this information into slopeintercept form:
 x represents the number of miles traveled by the rover.
 y represents the amount of battery power left. We need to find the numbers to replace m and b.
 The battery power decreases by 2 units each mile, so you should put −2 as the slope.
 Since the Rover starts with 20 units of battery power, 20 is the yintercept.
So how many miles can the Rover travel until the battery power gets down to 0 units? We put 0 in for y, the amount of battery power left. Now you can solve this equation to find x, the number of miles traveled:
 First, add 2x to both sides of the equation.
 Now you can divide both sides by 2.
And x = 10. So, this means that the rover can travel 10 more miles with it's power. It can easily reach the spot that is 8 miles away.
If you graph y = −2x + 20, you can see the solution right away: the xintercept is 10. This is the point where now power is left.
SlopeIntercept Form Example 2
Let's look at one more problem. The Rover has to get to the top of the mountain to F (7, 6). It can either take a route starting here at S1 (1, 0) or here at S2 (5, 0). The rover can go a maximum slope of 2.5. Which route is better?
Since the Rover can't go over anything with a slope higher than 2.5, we need to find the slope of two different routes. First let's find the slope of S2(5, 0) to F(7, 6).
The slope formula is y2 − y1 over x2 − x1. So, the first slope is 6 − 0 over 7 − 5. This reduces to 6 over 2 which is 3. That slope is too steep! Hopefully the other starting point will be better.
The second slope from S1(1, 0) to F(7, 6). Is 6 − 0 over 7 − 1. This reduces to 6 over 6 which is 1. This slope is better for the Rover.
It looks like we are pretty much done. But unfortunately, the Rover needs the whole formula to get into perfect position for the sunset. To find the yintercept, we can plugin all of the other information we know.
The slope, m, is 1. Now we can choose a point (x, y) on the route to plug in values for x and y. Let's choose the final point (7, 6). 6 goes in for y. 7 goes in for x.
Now we can solve for b. Subtract 7 from both sides. B equals negative 1. So the equation for the Rover's route is y = x − 1.
NASA managed to get the Rover in the perfect position for a romantic sunset. But what is this? Love is in the air on planet Mars.
SlopeIntercept Form Exercise

Describe the effects of changing $m$ or $b$.
HintsDraw the lines of
 $y=2x+2$ and
 $y=4x+2$
What can you see? If the slope changes, what happens?
The slope is:
$m=\frac{\Delta y}{\Delta x}$.
SolutionThe slopeintercept form is:
$y=mx+b$, where
 $m$ is the slope
 $b$ is the yintercept.
 if $m$ is positive (negative), the line increases (decreases).
 if the absolute value of $m$ increases (decreases), the line is steeper (less steep).
 if $m=0$, the line is parallel to the xaxis.
 this is the point where the line crosses the yaxis.
 if $b$ increases (decreases), the line lies higher (lower) in the coordinate system.

Decide whether or not the Mars rover has enough battery power left to drive eight miles.
HintsThe slopeintercept form is:
$y=mx+b$, where
 $m$ is the slope
 $b$ is the yintercept.
For each mile the rover travels the battery power decreases by 2 units.
Which means we'll have a negative slope.
The xintercept is at $y=0$.
SolutionThe rover starts with 20 units of battery power left. This is the yintercept. Because the rover needs two units of battery power each mile, the slope is $m=2$. Each mile the battery power decreases by 2 units.
This gives us the equation: $y=2x+20$.
We can determine the xintercept of this equation:
$\begin{array}{rcr} 0&=&2x+20\\ \color{#669900}{+2x} && \color{#669900}{+2x}\\ 2x&=&20. \end{array}$.
Now we divide by $2$ and get the xintercept at: $x=10$.
Therefore, the Mars rover has enough battery power left to drive eight miles ($8<10$).

Determine the lines and corresponding equations.
HintsThe slopeintercept form is:
$y=mx+b$
 $m$ is the slope
 $b$ is the yintercept.
First, have a look at the yintercept.
For the slope, calculate the change in $y$ divided by the change in $x$.
$m=\frac{\Delta y}{\Delta x}$.
SolutionThe yintercept of the blue, red. and violet lines is at $2$. The other two lines have the yintercept at $4$.
Let's have a look at the slope:
 The blue line: for each four steps to the right, we go one step down  this gives us the slope $m=\frac14$. This is the equation: $y=\frac14x+2$.
 The yellow line is parallel to the blue line, so the lines have the same slope $m=\frac14$. This is the equation: $y=\frac14x+4$.
 The red line: for each two steps to the right, we go one step up  this gives us the slope $m=\frac12$. This is the equation: $y=\frac12x+2$.
 The green line is parallel to the red line. So the lines have the same slope $m=\frac12$. This is the equation: $y=\frac12x+4$.
 The violet line: for each step to the right we go one step up  this gives us the slope $m=\frac11=1$. This is the equation: $y=x+2$.

Determine the equations which the describe the Mars rover's expedition.
HintsTo write the slopeintercept form you need:
 the slope $m$ and
 the yintercept $b$.
The slope is the change in $y$ divided by the change in $x$:
$m=\frac{\Delta y}{\Delta x}=\frac{y_2y_1}{x_2x_1}$.
Plug the x and ycoordinates of the given points into the definition of $m$.
Pay attention to the order of subtraction.
For the intercept you can
 use the pointslope form
 substitute one point into the slopeintercept form
SolutionIf you'd like to write the slopeintercept form of an equation with the two points ($P(4,0)$ and $Q(9,7)$):
 Calculate the slope by $m=\frac{\Delta y}{\Delta x}=\frac{y_2y_1}{x_2x_1}$
 Determine the yintercept with the slope.
$m=\frac{\Delta y}{\Delta x}=\frac{70}{94}=\frac75=1.4$.
Now we can write down the equation as follows
$y=1.4x+b$.
The yintercept
We substitute $x$ and $y$ with the given coordinates of one of the two points. It doesn't matter which point we choose. With the point $P(4,0)$  $x=4$ and $y=0$, we get the equation
$0=1.4\times 4+b$.
This gives us $b=5.6$.
This leads to: $y=1.4x+5.6$.

Decide which equations are in slopeintercept form.
HintsThe slope indicates the steepness of the line.
The yintercept is the point where the line crosses the yaxis.
The pointslopeform is:
$yy_1=m(xx_1)$.
SolutionThe slopeintercept form is:
$y=mx+b$, where
 $m$ is the slope. The greater the absolute value of the slope, the steeper the line.
 $b$ is the yintercept, the point where the line crosses the yaxis.
 $y=2x+20$ and $y=2x+1$ are in slopeintercept form.
 $y10=2(x5)$ is the pointslope form of $y=2x+20$.
 $m=\frac{\Delta y}{\Delta x}$ is the slope.

Determine the correct equation for the given points.
Hints$\Delta y=y_2y_1$ is the change in $y$, and $\Delta x=x_2x_1$ is the change in $x$.
If the yintercept is know, substitute the coordinates of any given point into the slopeintercept form to calculate the slope.
You can also check each the equation, do both points satisfy the equation?
Solution First, we calculate the slope: $m=\frac{\Delta y}{\Delta x}=\frac{y_2y_1}{x_2x_1}$
 Then, we determine the yintercept.
 $m=\frac{\Delta y}{\Delta x}=\frac{42}{43}=\frac21=2$.
 Next we substitute the coordinates of, for example $Q$, in the equation $y=2x+b$. Rewrite the equation as $4=2\times 4+b$. Subtracting $8$ we get $b=4$.
$\mathbf{P(1,2)}$ and $\mathbf{Q(0,1)}$
 $m=\frac{\Delta y}{\Delta x}=\frac{1(2)}{01}=\frac31=3$.
 Next we substitute the coordinates of, for example $P$, in the equation $y=3x+b$. Rewrite the equation as $2=3\times 1+b$. Adding $3$ we get $b=1$.
$\mathbf{P(4,6)}$ and $\mathbf{Q(2,5)}$
 $m=\frac{\Delta y}{\Delta x}=\frac{56}{24}=\frac12=0.5$.
 Next we substitute the coordinates of, for example $P$, in the equation $y=0.5x+b$. Rewrite the equation as $6=0.5\times 4+b$. Subtracting $2$ we get $b=4$.
$\mathbf{P(6,0)}$ and $\mathbf{Q(3,3)}$
 $m=\frac{\Delta y}{\Delta x}=\frac{30}{3(6)}=\frac39=\frac13$.
 Next we substitute the coordinates of, for example $Q$, in the equation $y=\frac13x+b$. Rewrite the equation as $3=\frac13\times 3+b$. Adding $1$ we get $b=2$.