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# Equation with two rational expressions (old example)

Sal solves the equation 4/(p-1)=5/(p+3). Created by Sal Khan and Monterey Institute for Technology and Education.

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• Is 0 positive or negative?
• 0 is neither positive nor negative it is just 0
• What does he mean by "undefined"?
• you can't divide any number by zero because you cannot multiply zero by any number to get the numerator, ever. saying undefined means that a problem would represent this kind of situation- when the problem just can't happen.
• At , why does he try to find the excluded values when there is only one answer to the equation?
• In general when you see an equation like this, it's not always easy to see what the solution is, but it is relatively straight-forward to find the values for which the fractions are undefined by having a zero in the denominator. This step is necessary because if you proceeded to solve this type of equation and the solution was indeed one of the excluded values, then this equation doesn't have a solution because the equation is undefined at this specific value.
• At the end of solving the equation, why did you multiply both sides by -1? I don't see where it came from if we cancelled out the -1 when multiplying both sides by (p-1).
• Because we had a -p = -17 but we want to know what p equals, not -p.

We had that -p because earlier in the steps taken we had subtracted 5p from 4p
• why don't you just cross-multiply?
• Cross-multiplication is a shortened version of what Sal did. It's the same method, excepting skipping a few steps and accounting for that.
(1 vote)
• Why is Sal not showing videos of rational expressions involving logarithms, exponents, roots and other operations? I think those rational expressions are just as important.
• Can anyone tell me how to solve a rational equation when there is a variable as the denominator? for example: 3/y + 4/3=-1
I understand how to solve rational equations but just not when there is a variable in the denominator.
• If you divide both sides of the problem (each individual part) by 3 you can isolate the Y. So you'd get y + 4/3/3 ( <--same as 4/3 x 1/3) = -1/3
Once you've done this move the ( 4/3 x 1/3) to the other side of the equal sign.
So the unsimplified answer would be y= -1/3 - ( 4/3 x 1/3)
(1 vote)
• can you help me plz on this problum 1/2-j+2=4/2-j