Subtracting Fractions with Different Denominators Trick - do them instantly! - By tecmath
Transcript
| 00:0-1 | Welcome to the Tech Math channel . What we're going | |
| 00:02 | to be having a look at in this video is | |
| 00:04 | how to subtract fractions from one other fairly instantly , | |
| 00:06 | it's going to cut out all this , messing around | |
| 00:09 | trying to find common denominators and things like this . | |
| 00:11 | And we're just going to directly subtract one fraction from | |
| 00:15 | another with different denominators . So let's just launch into | |
| 00:17 | this , Let's start out with an example here , | |
| 00:20 | we'll go 4/5 And from this , we're going to | |
| 00:23 | subtract 3/4 . Okay , now let's just put out | |
| 00:29 | answer over here . So the first thing we're gonna | |
| 00:32 | do is we're going to work out this denominator this | |
| 00:34 | number on the bottom and we're going to do this | |
| 00:37 | by multiplying here five times four is equal to 20 | |
| 00:41 | . I just multiply those guys . The next thing | |
| 00:44 | we're going to do is we're going to start in | |
| 00:45 | the upper left here on this first fraction And we're | |
| 00:48 | going to multiply it by the bottom number by the | |
| 00:50 | denominator on the second fraction . So we're going from | |
| 00:53 | top left to bottom right . And we're gonna multiply | |
| 00:55 | these four times four is equal to 16 . We've | |
| 01:00 | got the same symbol which is subtraction . And now | |
| 01:03 | we'll go over to the other part . This is | |
| 01:07 | This three here , and it's going to be multiplied | |
| 01:09 | by this five here . Okay , so three times | |
| 01:12 | five is equal to 15 . So you can see | |
| 01:14 | that bit of a pattern that we just use their | |
| 01:17 | . All right , what have we got ? 16 | |
| 01:18 | ? Take away 15 is equal to one and this | |
| 01:21 | goes over 20 . There's our answer . No . | |
| 01:24 | Working at common denominators saves a whole bunch of work | |
| 01:27 | . Right . What about we go through another example | |
| 01:30 | . Okay , for the next one . Let's get | |
| 01:32 | 7/8 . And from this we're going to take away | |
| 01:35 | 3/5 . All right . Just watch out when you | |
| 01:39 | use this in math class . It will put the | |
| 01:41 | wind up your teacher a little bit . They get | |
| 01:43 | where's all the working out ? But you know , | |
| 01:45 | you won't need to do all the working out . | |
| 01:46 | Right ? So let's just get our answer . Not | |
| 01:49 | mess around by working at common denominators . Okay , | |
| 01:52 | so first off , let's multiply these two denominators , | |
| 01:55 | eight times five is equal to 40 . The next | |
| 01:59 | thing we're going to do . We're going to start | |
| 02:00 | in the top left here and we're gonna multiply by | |
| 02:03 | the bottom , right ? Seven times five is 35 | |
| 02:06 | . We're going to subtract . The last thing we're | |
| 02:09 | going to do is we're going to go eight Times | |
| 02:12 | 3 3 , 8 , What do we got here | |
| 02:16 | ? So , let's see . 35 take away 24 | |
| 02:19 | is equal to 11 11/40 . There's our answer . | |
| 02:24 | Now , look , if you want to go through | |
| 02:25 | this , you can actually see what we're doing is | |
| 02:27 | we are finding common denominators and things like this . | |
| 02:29 | But we're skipping a bit of a step by working | |
| 02:32 | it all out individually . That sort of deal . | |
| 02:34 | We're just doing it in one fell swoop . So | |
| 02:37 | it's just a more efficient way of doing it . | |
| 02:39 | Okay , another example then I'll get you doing this | |
| 02:41 | for you Whatever we do . 5 9ths And we're | |
| 02:44 | going to subtract from that to over seven . All | |
| 02:48 | right . Usually a bit annoying to try and find | |
| 02:51 | common denominators here , but For us , easy nine | |
| 02:55 | times 7 is 63 . We're going to now go | |
| 02:59 | five times 775 35 the subtraction stays the same and | |
| 03:06 | two times nine is 18 . What do we have | |
| 03:10 | now ? Okay , 35 take away 18 is 17 | |
| 03:15 | And this is going to go over 63 . There | |
| 03:17 | is no common denominator . We're not reducing that any | |
| 03:20 | . So , what about I give you a couple | |
| 03:21 | of examples . I'm gonna give you ones that are | |
| 03:23 | a bit harder . Okay , let's go for fifth | |
| 03:28 | . And from this , we're going to subtract 2/3 | |
| 03:33 | the second one . I'm going to get you doing | |
| 03:35 | . Let's go 6/11 . And from this we're going | |
| 03:39 | to subtract one third . Hey , or some american | |
| 03:43 | . You can do these really quickly . I'm not | |
| 03:45 | gonna worry about the colors here . We don't need | |
| 03:47 | those anymore . I reckon we can do without it | |
| 03:48 | . Okay , five times 3 is equal to 15 | |
| 03:53 | . Let's see . We can do this directly . | |
| 03:54 | 12 take away . 10 Is equal to two . | |
| 03:59 | All right . Just skip the little step there . | |
| 04:02 | Right , Pretty clever , eh ? Let's go on | |
| 04:04 | this 1 . 11 times three is 33 . We're | |
| 04:09 | going to go 18 . subtract a lemon which is | |
| 04:12 | seven . All right , pretty cool . Right , | |
| 04:15 | How'd you go with it ? And you get the | |
| 04:17 | right answers If you like this video , please hit | |
| 04:20 | the like button and make a comment down below . | |
| 04:22 | Okay ? Uh , and if you want more tips | |
| 04:25 | and tricks on how to do fractions , if you | |
| 04:27 | look in the description below in this video , you're | |
| 04:30 | going to see , there is a playlist that playlist | |
| 04:32 | takes you to all sorts of fractional type videos , | |
| 04:34 | fractions of your fractions air , and you'll be up | |
| 04:37 | to your eyeballs Infractions . So anyway , thank you | |
| 04:40 | for watching and we'll see you next time . Bye | |
| 00:0-1 | . |
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Subtracting Fractions with Different Denominators Trick - do them instantly! is a free educational video by tecmath.
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