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Finding y intercept given slope & point (old)

An old video where Sal finds the y-intercept of the line y=17/13x+b given that the line passes through (5,8). Created by Sal Khan.

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• Where do you get the "24" from?
• I'm going to assume you mean his multiplication at . All he's doing is breaking up the operation 8*13 into groups to make it easier to do in his head. Given the distributive property:
``a(b+c) = ab + ac thus 8*13 = 8(10+3) = 8*10 + 8*3``

8 times 10 is 80, and 8 times 3 is 24. Add them together and you get 104. It's more steps than doing 8 times 13, but since they're on the times tables and well practiced/memorized, it's easier to do that way!
• still didn't make sense how did u get 104
• MHarmon,
8- 85/13 To subtract the fraction, you must first find a common denominator.
8/1 and 85/13 has a least common denominator of 13.
To change 8/1 to something over 13, you multiply by 13/13
(8/1)*(13/13) - 85/13 And multiply the numerators and denominators
(8*13)(1*13) - 85/13 And not 8*13 is 104 (which is where the 104 came from.
104/13 - 85/13 Now you can abstract the numerators
(104-85) / 13 =
19/13

I hope that helps!
• Is there any other way to get the y-intercept?
• In the Video, could you please show this work on a graph? To show the answer y-intercept b?
• What made him go from 8 = 85/13 + b to 8 - 85/13 = b, how come it was a subtraction symbol and not an addition symbol?
(1 vote)
• He moved 85/13 to the left side of the equation in order to get b alone. So he had to subtract it from both sides (because it's originally being added/positive).

So it becomes 8 - 85/13 = b
• Is there a video where I can figure out how to solve a problem like: Write the equation and give the y-intercept for the line that passes through the given points. (One example would be (4,2), (-3,9)
(1 vote)
• How did he get he number 8?
(1 vote)
• The problem statement gave Sal the point (5, 8).
This means x=5 and y=8
So, the 8 was part of the info given for the problem.
• Line AB has a slope of −4/5. If A (1,4) and B (x,12) , what is the value of x?
Please do not flag this, I know its a homework question but i looked everywhere to find the solution to these kinds of problems. I even asked my teacher, she tried to explain it but i didn't understand, however I told her I'll figure it out since other students had questions too but now obviously I cant figure it out so any help would be appreciated.
(1 vote)
• You can the method described in the 1st response. Or, you can also use the slope formula: m = (y2-y1) /(x2-x1)
-4/5 = (12-4)/(x-1)
Simplify: -4/5 = 8/(x-1)
You now have a proportion. Cross multiply to get: -4(x-1) = 5(8)
Then, solve for X.