Simplifying this algebraic fraction

Question

I don't understand how to simplify this fraction: $$\frac{12ab-12b}{8ac-24c}$$ This is my idea to what should be done, but I think it is totally wrong: $$\frac{12ab-12b}{8ac-24c}=\frac{12\cdot a\cdot b-12\cdot b}{8\cdot a\cdot c-24\cdot c}=\frac{12b^2-12b}{8c-24c}=\frac{b}{8c-24c}=?$$

Edit: I want to simplify the fraction.

Answer 1

In general

$$\frac{12ab-12b}{8ac-24c}\ne \frac{12b^2-12b}{8c-24c}.$$

You can divide $^1$ both numerator and denominator by $4=\gcd(12,8,24)$:

$$\frac{12ab-12b}{8ac-24c}=\frac{(12/4)ab-(12/4)b}{(8/4)ac-(24/4)c}=\frac{3ab-3b}{2ac-6c}.$$

If you want you can factor the numerator and denominator

$$\frac{3ab-3b}{2ac-6c}=\frac{3b\left( a-1\right) }{2c\left( a-3\right) }.$$

Since there are no common factors you cannot simplify further.

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$^1$ In greater detail:

$$\frac{12ab-12b}{8ac-24c}=\dfrac{\dfrac{12ab-12b}{4}}{\dfrac{8ac-24c}{4}}=\dfrac{ \dfrac{12ab}{4}-\dfrac{12b}{4}}{\dfrac{8ac}{4}-\dfrac{24c}{4}}=\dfrac{\dfrac{12}{4} ab-\dfrac{12}{4}b}{\dfrac{8}{4}ac-\dfrac{24}{4}c}=\dfrac{3ab-3b}{2ac-6c}.$$

Answer 2

I have several bits to say here.

Firstly, the phrase 'solve' implies an equation, something like $\text{expression} = \text{expression}$. Here, we just have an expression, so there is no equation to solve. I suspect you mean to 'simplify' the fraction.

Secondly, let's talk about what things are you allowed to do to simplify a fraction? You tried $$\frac{12ab-12b}{8ac-24c}=\frac{12*a*b-12*b}{8*a*c-24*c}=\frac{12b^2-12b}{8c-24c}=\frac{b}{8c-24c}=?$$ However, this is not correct. In particular, in the middle two expressions, I noticed that you cancelled the a's from the left numerator and left denominator. This would be like saying that $3 = \dfrac{8 + 7}{4 + 1} = \dfrac{2 + 7}{1 + 1} = \dfrac{9}{2}$. That's just not true. I'll return to this in my third point. Before that, I wanted to talk about one more problem: in the last two expressions, you wrote $12 b^2 - 12b = b$. This is also not correct. For a concrete example, we look at $12 \cdot 3^2 - 12 \cdot 3 = 72 \not = 3$. I'm a bit uncertain as to what to say to that, except that it's very not allowed. Though it's not useful here, I do mention that you could factor that expression: $12b^2 - 12b = 12b(b - 1)$. But that's really all there is to do there.

Thirdly, how should you actually proceed? One way is to factor the top and bottom as much as you can. For example, $12ab - 12b = 12b(a - 1)$ and $8ac - 24c = 8c(a - 3)$. Rewriting my main fraction, we get $\displaystyle \frac{12ab - 12b}{8ac - 24c} = \frac{12b(a-1)}{8c(a-3)}$. Then I note that we could rewrite $\dfrac{12}{8}$ as $\dfrac{3}{2}$. But there is no more simplification to be done.

Finally, I note that some people consider factoring to be a simplification. I do not, and I get tired when grading homework or tests when students arbitrarily start factoring expressions into things that I consider far less simple - but the main point is that different teachers have different expectations. Make sure you are aware of those.