Complete each statement.
1. Find the missing constants so each equation is true for every allowed value of the variable.
a) Find \(r\) and \(s\) so \((rx+sy)\div(x+y)=13\), where \(x+y\ne0\).
b) Find \(c\) so \((15z-c)\div(3z-4)=5\), where \(3z-4\ne0\).
2. A student claims, “\((ax+ay)\div(x+y)\) always equals \(a\), no matter what values are used for \(x\) and \(y\).” Explain why the claim needs a restriction. For which values does the quotient not exist?
Hints
- Work backward by multiplying the desired quotient by the denominator.
- Compare corresponding coefficients after expanding.
- Determine exactly when the denominator of the claimed quotient equals zero.
Solution
1. For the first identity, the numerator must equal \(13(x+y)=13x+13y\), so \(r=13\) and \(s=13\).
2. For the second identity, \(5(3z-4)=15z-20\), so \(c=20\).
3. Since \(ax+ay=a(x+y)\), the quotient equals \(a\) only when \(x+y\ne0\). If \(x+y=0\), equivalently \(x=-y\), the denominator is zero and the quotient is undefined.
Answer
1. a) \(r=13\), \(s=13\)
b) \(c=20\)
2. The claim is valid only when \(x+y\ne0\). The quotient is undefined when \(x=-y\).