New Multiplication A

How to multiply 2 double-digit numbers
whose tens digits are both one (1).

14 x 17 = (10 + 4)(10 + 7)
= 100 + 70 + 40 + 28
= 238

19 x 15 = (10 + 9)(10 + 5)
= 100 + 50 + 90 + 45
= 285

The pattern is:

1a x 1b = (10 + a)(10 + b)
= 100 + 10b + 10a + ab
= 100 + 10(a+b) + ab
= 100 + 10(sum of units) + product of units

And the above two examples can now
be calculated mentally as follows -

14 x 17 = 100 + 10(4+7)+ 28
= 100 + 110 + 28
= 238

19 x 15 = 100 + 10(9+5)+ 45
= 100 + 140 + 45
= 285

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