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Course: Algebra basics>Unit 4

Lesson 7: Writing slope-intercept equations

Slope-intercept form problems

Learn how to solve problems involving writing an equation in slope-intercept form.  Created by Sal Khan.

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• Isn't double minus sign incorrect? Shouldn't be there parentheses? Like 5 - (-2) instead of 5 - -2
• Yes, there should be parentheses separating the minus signs.
• How do I write the slope-intercept form of the equation of the line through: (3,-2), if the slope is undefined?
• Hi,
If a slope is undefined, it means that the x value does not change. Remember that slope is calculated as the change in the y value divided by the change in the x value. If x does not change, then we are dividing by zero, which is undefined. Therefore, an undefined slope is a vertical line through the point you were given.
You cannot really write it in a correct slope/intercept form. Usually this is written as:
x=whatever the x value of your point is.
In your case, the equation would be
x=3
And the graph would be a vertical line running through the 3 on the x axis.
Hope that helps :-)
• At , when you had 0=5/6(-3)+b, why did you divide by 3 instead of multiply? And why only the 3 and 6 and not the 5?
• To multply 5/6 (-3):
Change -3 into a fraction: 5/6 (-3/1)

You now have 2 choices:
Option 1) You can multiply the 2 fractions. Multiplying fractions means you multiply numerator * numerator and denominator * denominator. Then reduce.
5/6 (-3/1) = -15/6 = -5/2

Option 2) You can reduce 1st. Reducing cancels out a common factor from the numerator to the denominator. As long as fractions are being multiplied, you can cancel across fractions. This is what Sal did. Sal cancelled out a factor of 3 from -1 and 6.
5/6 (-3/1) = 5/2 (-1/1) = -5/2

Hope this helps.
You may want to review multiplying and reducing fractions.
• I didn't understand the equation f(1.5)=-3,f(-1)=2?
• This is because our function here is
f(`x`)=(`x`)-2
So each time we put in a value of `x` we multiply it by -2
Ex.
f(`1.5`)=(`1.5`)*-2=-3
f(`-1`)=(`-1`)*-2=2
• mane dis to complicated ion know how to do dis
• Fr I don't understand :/
• there is some website that we can take 2 random points and it'll compute the equation for us, i guess
• Yes,it's called a calculator.
• The question was asked thatg the line contains two points. You took 0-5 and -3-3. The other question was f(1,5) = -3, f (-1) = 2. You subtracted 2- (-3). How do i know which side to subtract. How do i know what y2 , y1 , x1 and x2 is?
• It doesn't matter which point you may (x1, y1) vs (x2, y2).
If you do the math correctly, you will get the same slope.
Just pick one point and label it (x1, y1). Next, label the other as (x2, y2). Then, plug the values into the slope formula.
• 0= 5/3(-3) + b why did sal divide -3 by -3 to get a +ve 1?
• Sal did not divide -3 by -3. They were dividing the 0-5.
• At , Sal divides -3 by 3 and then -3 by 6. Why does he divide -3 by 3 and -3 by 6, but not -3 by 5?
• Sal is multiplying: (5/6)(-3) = -15/6
When we reduce fractions, we remove any common factors from the numerator and denominator. The common factor is 3. The simplifies the fraction into -5/2.

Sal chose to cross cancel (cancel out the common factor, then complete the multiplication.

Hope this helps.