multiplication trick

>>8849840
common core?
(10-2)*8
(5+2)*8
(5+2)*7

>>8849840
did you memorize how to speak your mother tongue? No, you just grew up hearing it and knowing how it feels to speak it.

You just had to learn how 8*8 feels like 64, 7*8 feels like 56, 7*7 feels like 49.

>without memorizing
Don't talk stupid.
Memorise the square numbers.

7x8=8x8-8

>>8849844
Otherwise this.
But seriously just learn the squares.

>>8849847
b-but
9 * 9 can be shortened to 90 - 9
9 * 6 can be (9 - 1) / 2 and add 5 on first digit

>>8849851
not him but you mean memorize 5**2 , 6**2, 7**2 etc?

>>8849855
i meant 9 * 5

add number on left to itself a number of times equal to number on right
e.g.
8 * 7 = 8 + 8 + 8 + 8 + 8 + 8 + 8 = 56

>>8849867
something compact
why would i add if i can remember table in my head

>>8849851
i guess memorizing is the only way out

>>8849859
You mean 25, 36, 49 etc?
Yes.

Obviously you can't memorise them all, but is makes mental arithmetic a lot easier if you just know a bunch of results without having to calculate them.

Squares are useful so worth memorising.

>>8849855
I understand, that might be the first step.

If you're interested in why that is, you should watch this excellent video

https://www.youtube.com/watch?v=UBVV8pch1dM

For know, your brain will follow instructions, but the next step is making it intuitive so that it costs less mental "stamina".

>>8849877
good vid anon
i am trying to get highscore in brain age 3ds

(must play if someone is having problem concentrating)

>>8849844
this is a good way to solve small prob easily

For multiplying larger numbers in your head, here's what I do:

Find the nearest easy to work with number to each, like a multiple of a power of ten. Call these a and b.
Take the difference between those numbers and the originals, call these c and d. Then do this:

(a-c)(b-d) = ab - 2(ad + bc) + cd.

If you choose a and b well, this should be pretty simple. Also make sure to account for the signs of c and d.

>>8849939
Whoops, put a 2 in there because I usually use this for squaring things. Ignore the 2.

>>8849939
can you give a example ?

>>8849944
93 * 47 = 100*50 - 100*7 - 50*3 + 21
So 93 * 47 = 5000 - 700 - 150 + 21 = 4171.

>>8849949
another good trick anon
took me some time to understand

>>8849949
93 *47 = 4371

(a-c)(b-d) = ab - ad + b + cd.

>>8849840
using a calculator

>>8849978
Ah, you got me there. In my defense, I haven't slept in about 2.5 days, not that that matters.

>>8849978
It should be

93*47 = 100*50 - 100*3 - 50*7 + 21
93*47 = 5000 - 300 - 350 + 21 = 4371.

>>8849840
When I was younger I always had trouble meme-orizing the "late mid" tables, i.e. 6*7, 7*8, and hell, even the additions, 6+7, 7+8. I just had to grind that out. One way I tried to help remember was for instance for 7*8, get 6*8, 48, and add 8, 56.

>>8850016
go get a good sleep
knowledge without sleep not worth it

(a-c)(b-d) = ab - ad + b + cd was also incorrect
correct is
>>8850020

irrelevant to the topic but is this correct:√245 = √49*5 = 5√7
i understand the concept but i can't figure out an easy way to figure out the middle part

>>8849840
I don't remember ever memorizing these, yet I have them completely ingrained into my brain.

As >>8849847 said, similar to a language.

>>8850020
91*44 = 100*50 - 100*9 - 50*6 +54
91*44 = 5000 - 900 - 30 +54
91*44 = 4124
but
91*44 = 4004

what am i doing wrong

>>8850055
you did too much math and now you can understand things others cant
just like musicians can tell which note is being played just by hearing

idk how it works on maths

>>8850067
Same thing I did. Multiply the new numbers by the difference between the other pair. In your example it would be:

91*44 = 100*50 - 100*6 - 50*9 + 54

>>8850089
that solved it

>>8849840
are you...black ?

>>8850111
indian
i guess its the same thing

Oh, another good trick I sometimes use involves the classic "multiply by nine using your fingers" trick. If you want to find a*b, you can pretend you have a+1 fingers, imagine bending down the bth one from the left, then set the number remaining to the left and right as your first and second digits. Then you just convert to base 10 and you're done.

>>8850128
Note that this requires you to choose a to be the larger of the two numbers.

70/3.6 = 70/(36/10) = 70/(18/5) = 70/18*5 = 70/2/3/3*5 = 35/3/3*5 = (36-1)/3/3*5 = (12-1/3)/3*5 = (4-1/9)*5 = 20-5/9 = 19.444...

>>8849840
2*2*2**2*2*2
7**2*2*2
^that number minus 7

So long as you know 8 is 2*2*2, its easy. Idk any other way to avoid memorization because math is entirely memorization.

>>8850023
When exactly did you have to "grind that out" this is shit Americans know by heart by age 10