### Video instructions and help with filling out and completing What Form 8815 Subtract

**Instructions and Help about What Form 8815 Subtract**

In this lesson we're gonna focus on adding and subtracting fractions let's say if we have 3/5 plus 4/7 how can we add these two fractions well here's a simple technique first multiply five and seven this will give you a common denominator of 35 next multiply three and seven three times seven is twenty-one and also there's a plus in between multiply 5 times 4 which is 20 21 plus 20 is 41 so the answer is 41 divided by 35 let's try another example let's say if we want to subtract 7 over 8 minus 2 over 9 let's use the same technique let's multiply the two denominators 8 + 9 which is 72 and then the next one it's gonna be 7 times 9 which is 63 minus 8 times 2 which is 16 now let's subtract what is 63 minus 16 this is gonna be 47 now 47 is not divisible by 2 nor is it divisible by 3 so this is it that's the final answer so now you know how to add or subtract two fractions now what if we wanted to add or subtract let's say three fractions instead of two what should we do in this case so let's say we wish to combine 3 over 4 plus 5 over 3 minus 7 over 2 whenever you wish to add or subtract fractions the denominator has to be the same the denominator is the bottom part of the fraction and right now they're all different so how can we make them the same how can we get the commenter down there if you want to find the least common denominator Nikolas all of the multiples of 2 are 2 4 6 8 10 12 14 and so forth multiples of 3 are 3 6 9 12 15 18 and so forth and multiples of 4 are 4 8 12 16 it's 1 what is the least common multiple we're looking for a multiple that is common to all three numbers but it's the lowest the least common multiple is 12 12 is common to 2 3 & 4 and it's the lowest of such numbers now granted 24 is also a common multiple and if you use 24 you can get the right answer you just gotta simplify at the end so if you ever like unsure about how to find the least common denominator you can find any common denominator one simple technique is simply to multiply these 3 4 times 3 times 2 is 24 and you could use 24 and still get the right answer so now that we know the least common denominator is 12 let's multiply each fraction in such a way to get 12 the first fraction let's multiply the top and the bottom by 3 because 3 times 4 is 12 the second one let's use 4 and for the last one let's use 6 so looking at the first one 3 times 3 is 9 3 times 4 is 12 4 times 5 is 20 4 times 3 is 12 7 times 6 is 42 2 times 6 is 12 now that we have the same denominator we can combine the numerators 9 plus 20 is 29 and 29 minus 42 is negative 13 so this is the final answer it's negative 13 divided by 12 so now it's your turn go ahead and try this example 8 over 5 minus 2 over 3 plus 9 divided by 4 so go ahead and add these three fractions so this time to find the least common multiple we're just going to multiply 5 3 & 4 it may not be the least common multiple but it is a common denominator just so you know if we multiply 5 times 4 times 3 this will give us 5 times 4 is 20 20 times 3 is 60 so 60 is going to be the common denominator that we're gonna try to get so we're gonna multiply the first fraction by 12 because 12 times 5 is 60 and the second one by 20 because 20 times 3 is 60 by the way if you want to find out the number divide it 60 divided by 5 will give you the 12 60 divided by 3 will give you the term and 60 divided by 4 will give us the number that we'll need to multiply this fraction by which is 15 now 12 times 8 that's 96 5 times 12 sixty-two times twenty is 43 times 20 is 60 and nine times 15 15 times 10 is 150 so if you take away 15 from that you'll get 15 times 9 so that's 135 and 4 times 15 is 60 now 96 minus 40 that's positive 56 and 56 plus 135 let's go ahead and add those two numbers deal fashionably 5 plus 6 is 11 carry over the one one plus three plus five is nine plus one so the final answer is 191 divided by 60 in this lesson we're going to focus on multiplying two fractions whenever you need to multiply multiply the numbers across the fractions three times seven is equal to 21 and five times two is equal to ten and so this is it the answer is 21 over ten that's all you need to do when multiplying fractions but sometimes the numbers may not be that small let's say if we have large numbers well should we do in this case now we can multiply across we can multiply 24 and 45 which will give us a big number but do we really want to do that and we're multiplying fractions with large numbers it's your best interests to break down the large numbers into small numbers for instance 24 is basically 6 times 4 27 is 9 times 3 45 is 9 times 5 and 30 is 6 times 5 you want to break it in such.