Answer:
15. x = 4
16. a = 19
Step-by-step explanation:
"Cross multiply" essentially means "multiply both sides of the equation by the product of the denominators."
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15.
[tex]\dfrac{x}{6}=\dfrac{2}{3}\\\\3(x)=6(2)\qquad\text{"cross multiply"}\\\\\boxed{x=4}\qquad\text{divide by 3}[/tex]
This is an example of a problem in which cross-multiplying introduces the need for an extra step: divide by 3. Had we simply multiplied by the denominator of the x-term, we would have been finished in one step:
x = 6(2/3) = 4
That is, rather than multiplying by the product of the denominators, sometimes the problem can be solved more easily if you multiply by their least common multiple.
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16.
[tex]\dfrac{a+1}{a+5}=\dfrac{a-4}{a-1}\\\\(a-1)(a+1)=(a+5)(a-4)\qquad\text{"cross multiply"}\\\\a^2-1=a^2+a-20\qquad\text{simplify}\\\\\boxed{19=a}\qquad\text{add $20-a^2$}[/tex]
