Posted by **Wanda** on Sunday, July 22, 2012 at 4:04pm.

What is the process to list all numbers for which rational expression is undefined? Ex: y-3divided y+5

- math -
**Reiny**, Sunday, July 22, 2012 at 4:29pm
Look to see if the expression contains any fractions.

Those fractions cannot be zero, since we cannot divide by zero.

So take each of the fractions and set them equal to zero and solve.

You now have the value of the variable that we **cannot** use.

in your example,

(y-3)/(y+5)

y+5 = 0

y = -5

so:

(y-3)/(y+5), y ≠ -5

in some cases the could be more than one restriction.

e.g. 5x/(x^2 - x - 6)

x^2 - x - 6 = 0

(x-3)(x+2) = 0

x= 3 or x = -2

So we have :

5x/(x^2 - x - 6) , x ≠-2, 3

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