Same principle as multiplying the common way, but with lines. 13x12 is (10+3)x(10+2), which you can expand as 100+30+20+6. So, in the first picture, you have 1 intersection on the left, 2+3 intersections in the middle, and 6 intersections on the right. Job done.
I'm just guessing, but I'm pretty sure you take the digits and make a certain amount of lines of that digit (5=5 lines, 6=6 lines, etc.). You do the first digit of the first number, then the first digit of the second number, then the second digit of the first number and so on. You make the first lines horizontal and the second lines vertical. Then the third lines are horizontal and the fourth lines are vertical. You look to see how many times they cross (lines cross twice=2, lines cross 3 times= 3) by going over it clockwise. Then those numbers you get are your answer. I only did this by studying the first picture. If anybody actually knows, please correct me.
Cofession: when in any class and I get bored I sit there and do multiplication in my head just to see how close i get without a calculator and how quickly I can do it. The quickest I remember were multiplying 2 digit numbers by 2 digit numbers (I.e. 50x50) and could solve in about 20-30 seconds. I don't know why but I felt like sharing this because of this post
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· 9 years ago
I figure the powers of 2 and memorize them. Like 2^14 is 16,384 and I just know that now.
Let's try 12 x 13- it might be easier to actually do it so grab your paper and pen!
Draw vertical lines in a square shape- one line on the left, two on the right, representing 12 and 1 at the top, three at the bottom to represent 13.
The lines cross each other at the corners. In the top left corner you have 1 cross (first digit of answer), in the bottom right six (last digit of answer). For the rest you have to total all the crosses in the top right AND bottom left- in this case 5. Gives the answer 156.
Draw vertical lines in a square shape- one line on the left, two on the right, representing 12 and 1 at the top, three at the bottom to represent 13.
The lines cross each other at the corners. In the top left corner you have 1 cross (first digit of answer), in the bottom right six (last digit of answer). For the rest you have to total all the crosses in the top right AND bottom left- in this case 5. Gives the answer 156.