2 by 2 multiplication help

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#1 12 June, 2016 - 22:46
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Joined: 2 years 2 months ago

2 by 2 multiplication help

Hello everyone.

I have 10 days left preparing for a show in a college. I was planning to do some rapid calculations. So I was learning the easiest like square root cube root fifth root, day of the week, memorising square and cubes of 2 digit number using major system. I was thinking if I can do the 2 by 2 multiplication. I was planning on calculating it normally -
78*83
70*80=5600
8*8+7*3=85
8*3=24
6474

I am memorising the 56 85 24 by using major system.
What I have to do to do it within 5 seconds? O am planning on doing 100 multiplication a day. Is it enough? What I have to do?

is it possible?

13 June, 2016 - 10:14
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Joined: 1 year 5 months ago

I am memorising the 56 85 24 by using major system.

I don't understand why you are memorizing partial products. Why not memorize the final product?

Also, if you are going to do 2x2 from memory there are almost 5000 combinations to memorize

13 June, 2016 - 10:52
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Joined: 5 years 9 months ago

78 x 83 =
78 x 82 +78 =
80^2 - 4 + 78 =
6400 +74 =
6474

Why the need to memorize?

13 June, 2016 - 11:02
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Joined: 5 years 9 months ago

Also; if you must use criss cross multiplication, use '80 - 2' for 78.
Because then the steps become:

80 x 80 = 6400
-2 +3 = +1 x 80 = 80 + 6400 = 6480
-2 x 3 = -6 + 6480 = 6474

Again; no need to memorize, imho.

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