# Need some help

• Sep 13th 2009, 07:03 PM
Math Noob
Need some help

does anyone have a good tutorial so i can learn problems like this one?

18/13-10/17+3/2=?
• Sep 13th 2009, 07:10 PM
pickslides
$\displaystyle \frac{18}{13}-\frac{10}{17}+\frac{3}{2}$

Find a common denominator, the easiest way to find one is to multiply all the denominators together.

$\displaystyle 13\times 17 \times 2 = 442$

Now

$\displaystyle \frac{18\times17\times 2}{442}-\frac{10\times13\times 2}{442}+\frac{3\times17\times 13}{442}$

Can you finish it from here?
• Sep 13th 2009, 07:17 PM
Math Noob
I can i guess, but i would like to know why are you cross multiplying to get the answer? Is that what your suppose to do? I am in need of a tutorial explaing why the problem is solved that way.
• Sep 13th 2009, 07:27 PM
pickslides
Do you understand why the denominators need to be the same?
• Sep 13th 2009, 07:39 PM
Math Noob
yes i know how to make the denominators the same by multiplying the prime factors if they are not common factors what i don't understand is why are you multiplying the numerators to the denominators by cross multiplying.
• Sep 13th 2009, 08:14 PM
pickslides
Well lets take the first term as a case study.

$\displaystyle \frac{18}{13}$

As you understand we need to make the denominator 13 into 442. This is done by multiplying by 17 and 2. I.e 13 by 17 by 2 = 442.

So if we are changing the demoninator by multiplying by 17 and 2, then we must also do this to the numerator.

$\displaystyle \frac{18}{13} = \frac{18\times 17 \times 2}{13\times 17 \times 2 } = \frac{18\times 17 \times 2}{442 }$

Now you need to do this to the 2nd and 3rd term.
• Sep 13th 2009, 08:25 PM
Math Noob
i see that you have to multiply the denominator with the numerator but why is the 13 left out of the problem? 18x17x2
• Sep 13th 2009, 08:57 PM
pickslides
We didn't multiply the denominator by 13 so we don't times the numerator by 13 either, maybe this working will illustrate it better.

We are doing exactly the same to each the numerator and demoninator.

$\displaystyle \frac{18}{13} = \frac{18}{13}\left(\frac{\times 17 \times 2}{\times 17 \times 2 }\right) = \frac{18\times 17 \times 2}{442 }$
• Sep 13th 2009, 10:02 PM
Math Noob
You multiplied all the denominators together to get the final denominator.

So what is left is to multiply the numerator correct?

What the question i am referring to is the numerator, I can't get a grasp on why your multiplying. Sorry if I'm asking so many questions' but i am using this to write a study guide for myself, since i am home schooling. That is why i asked if their are tutorials on problems such as the one i presented with a in depth write up.
• Sep 13th 2009, 10:10 PM
pickslides
Quote:

Originally Posted by Math Noob
You multiplied all the denominators together to get the final denominator.

So what is left is to multiply the numerator correct?

I think I agree with what your saying

$\displaystyle \frac{18}{13}-\frac{10}{17}+\frac{3}{2} = \frac{18}{13}\left(\frac{\times 17 \times 2}{\times 17 \times 2 }\right)-$ $\displaystyle \frac{10}{17}\left(\frac{\times 13 \times 2}{\times 13 \times 2 }\right)+$ $\displaystyle \frac{3}{2}\left(\frac{\times 17 \times 13}{\times 17 \times 13 }\right) = \frac{18\times17\times 2}{442}-\frac{10\times13\times 2}{442}+\frac{3\times17\times 13}{442}$

Does that help?

Quote:

Originally Posted by Math Noob

What the question i am referring to is the numerator, I can't get a grasp on why your multiplying. Sorry if I'm asking so many questions' but i am using this to write a study guide for myself, since i am home schooling. That is why i asked if their are tutorials on problems such as the one i presented with a in depth write up.

I don't know of any tutorials to help you with this problem but I bet there is one. I think you should google "Addition of fractions" and see what you get.

(Rofl)
• Sep 13th 2009, 11:04 PM
Math Noob
Quote:

Originally Posted by pickslides
I think I agree with what your saying

$\displaystyle \frac{18}{13}-\frac{10}{17}+\frac{3}{2} = \frac{18}{13}\left(\frac{\times 17 \times 2}{\times 17 \times 2 }\right)-$ $\displaystyle \frac{10}{17}\left(\frac{\times 13 \times 2}{\times 13 \times 2 }\right)+$ $\displaystyle \frac{3}{2}\left(\frac{\times 17 \times 13}{\times 17 \times 13 }\right) = \frac{18\times17\times 2}{442}-\frac{10\times13\times 2}{442}+\frac{3\times17\times 13}{442}$

Does that help?

I don't know of any tutorials to help you with this problem but I bet there is one. I think you should google "Addition of fractions" and see what you get.

(Rofl)

Wonderful break down, you just opened my eyes'