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Start with the given expression
Set the denominator equal to zero. Remember, dividing by 0 is undefined. So if we find values of r that make the denominator zero, then we must exclude them from the domain.
Factor the left side (note: if you need help with factoring, check out this solver
Now set each factor equal to zero:
Now solve for r in each case
So our solutions are
make the denominator equal to zero, this means
make the rational expression undefined (remember you cannot divide by zero)