Calculate faster than a calculator

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Trick to multiply faster than a calculator.

If you have 2 numbers of two digits, where the first digits is the same in both numbers, then you can solve the multiply in this way:

x = a \times 10 + b
y = a \times 10 + c where b + c = 10

Then:

x \times y = [(a+1) \times a ] \bigcup [b \times c]

For Example:

64 \times 66 = (7 \times 6) \bigcup (6 \times 4) = 4224
51 \times 59 = (6 \times 5) \bigcup (1 \times 9) = 3009
83 \times 87 = (9 \times 8) \bigcup (3 \times 7) = 7221

and so on:

124 \times 126 = (13 \times 12) \bigcup (6 \times 4) = 15624
114 \times 116 = (12 \times 11) \bigcup (6 \times 4) = 13224

If you have questions you can see this video:

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