# [SOLVED] need to brush up on math skills

• Aug 12th 2005, 10:09 AM
mp123
[SOLVED] need to brush up on math skills
how do you subtract fractions?
• Aug 12th 2005, 11:28 AM
rgep
Briefly, to subtract a/b from c/d we first write them in a consistent way. For example, a/b = 2a/2b = 3a/3b = ... and c/d = 2c/2d = ... . What we want to do is find a common denominator (bottom line). So a/b = ad/bd and similarly c/d = bc/bd. So a/b - c/d = ad/bd - bc/bd. But now the fractions are consistent: that is, both are expressed as a number of (bd)-th parts. So we can simply subtract the number of parts, to get (ad-bc)/bd. There may be a common factor on both top and bottom, and if so we should remove it by dividing through.

Example: 3/4 - 2/5. 3/4 = (3x5)/(4x5) = 15/20 and 2/5 = (2x4)/(5x4) = 8/20. The difference between 15 twentieths and 8 twentieths is 7 twentieths, ie 7/20. In this case there is no common factor and 7/20 is in lowest terms.
• Dec 16th 2008, 05:16 PM
saranya
Quote:

Originally Posted by mp123
how do you subtract fractions?

hi
To subtract, First u have to see whether it is like fraction or unlike fraction.
Then, if it is like fraction directly u can subtract.
If it is unlike fraction u have to find LCM to make denominators to be same, then subtract it
ok
• Dec 16th 2008, 05:19 PM
Chris L T521
Quote:

Originally Posted by saranya
hi
To subtract, First u have to see whether it is like fraction or unlike fraction.
Then, if it is like fraction directly u can subtract.
If it is unlike fraction u have to find LCM to make denominators to be same, then subtract it
ok

I doubt that the person who asked this question will see this since this thread, after all, is over 3 years old...