How to git gud at mental math and basic arithmetic?

[Anonymous No.11283659]

How do you solve something like 12*14 in your head? Do you do:
>10*14 + 2*14 =168
or do you have some other way of doing quick maths. I just realized after all this time I've been so dependent on a calculator that I've almost forgotten how to do basic arithmetic without one

[Anonymous No.11283667]

>>11283659
i went...

12 to 14 12 to 14 12o to 14.. shit am i too slow? ... 12 to 14 ah 12 x12 is 144 cuz memory then 2x12 more is just 24 so 168 sum

[Anonymous No.11284093]

14×2=28

14×10=140

140+28=168

[Anonymous No.11284216]

Split up the numbers.
12x14 = (10+2)x(10+4)=10x10 + 2x10 + 4x10 + 2x4 = 100 + 20 + 40 + 8 = 168

[Anonymous No.11284224]

Yeah basically it's about knowing all the possible ways to do it. You need different stuff for different calculations.

[Anonymous No.11284379]

>How do you solve something like 12*14 in your head?

13^2-1^2 and I remember that 13^2 is 169

[Anonymous No.11284384]

Most people factor the numbers as some anons already said, if you want to get good at it just practice a lot, e.g. whenever you are bored think of two numbers and try to multiply them in your head.

[Anonymous No.11284397]

>>11283659
It’s all about finding the best shortcut.
First method I went to for this problem is 6*7*4, but it’s all about finding what works for you.

[Anonymous No.11285994]

12^2=144
12*2=24
144+24=168

[Anonymous No.11285998]

>>11283659
I went 120+48(120+40)(168).

[Anonymous No.11285999]

>>11285998
Just be like Warren Buffet and do quick maths every minute. What is 60% of 1400?

[Anonymous No.11286002]

>>11285999
6*10+24=84
84000/100
840i go like this in 30 secs max.

[Anonymous No.11286161]

I would have done 12*12 + 12*2

I just break it up in my head, you have to have learned times tables pretty young

[None !AnOnYMous2 No.11286191]

>>11283659
>>10*14 + 2*14 =168
Ya that or 12*12 which is already know is 144 + 24
Fuck doing that in your head. Just always have access to a calculator. It's not like you'd forget how to do this in your head. You're just psyching yourself out.

[Anonymous No.11286286]

>>11283659
13 * 13 - 1
BOW

[Anonymous No.11286298]

>>11286286
same

[Anonymous No.11286303]

>>11285999
Seems like a long way to go about it, but
50% of 1600= 700
700/5= 140 (another 10%)
700+140 = 840

I think I have the same problem as OP. Why did my mind instinctively go through that slower process when
50%=700
10%=140
700+140=840

[Anonymous No.11286307]

Any good books for this? I know about 'be a math magician!' but it's kind of cheese/overly wordy. I also know about MIT's art of estimation book.

[Anonymous No.11286308]

>>11284379
>>11286286
Does this work for all numbers around squares?
4*6 = 5^2 -1 = 24
19*21 = 20^2 -1 = 399
Huh. Holds up so far anyway..

[Anonymous No.11286310]

>>11285999
60% * 1400 == 1400 * 0.6 == 140 * 6 == 140 * (3 * 2)
140 * 3 = 420,
420 * 2 = 840

[Anonymous No.11286314]

>>11286308
yes
[math]x^2 = (x-1) * (x+1) +1[/math]

[Anonymous No.11286320]

>>11283659
12*12=144
+
12*2=24
24+144=168

[Anonymous No.11286325]

[math]==

[Anonymous No.11287216]

>>11286308
It does. Even something like 93*107 becomes an easy 10000-49.

[Anonymous No.11287232]

fun thing about squares, if you wanna find the square of 13 and you only know the square of 12, just get the square of 12,144 and add 1 + 2*12, this, ofcourse, is because the second difference of the nth term sequence is n^2 is 2 and the difference between 0^2 and 1^2 is 1, so you add 1 (first difference) + 2(second difference) * the amount of the second difference has been applied

[Anonymous No.11287245]

>>11283659

12*15-12

[Anonymous No.11287254]

>>11287216
So the general rule would be?

[math]x^2 = (x+y)(x-y) + y^2[/math]

[Anonymous No.11287439]

>>11287254
Yes!

[Anonymous No.11287495]

>>11283659
Hey OP, here's a git gud for you.
In case the math went too quickly, check the digital sum and the last one or two digits.

For example: 214+783+965=1857 Is it correct?

2+1+4=7----------------->7
7+8+3=18-->1+8=9-->0
9+6+5=20-->2+0=2-->2
The sum total of 7, 0 and 2 makes 9, which you can count it as zero. The digital sum of 1857 is 3 which means the whole sum is wrong.

If it were correct, the last two digits must be _14+_83+_65=_62. So the actual sum has to be 1962 to match the digital sum of zero it is expected to be (it's not 1862 because the digital sum is only 8).