Multiplication
of two digit numbers
When tens digit
are same and the sum of unit digits is 10
57x53 = 5 x 6 (100) + 21 = 3021
44 x 46 = 202462 x 68 = 4216
31 x 39 = 1209
84 x 86 = 7224
How
it works
ab x cdwhere
c=a
b+d=10
(a x10 + b)(c x 10 + d)
(a x 10 + b)( a x 10 + 10 - b)
100 a2 + 100a – 10 ab + 10 ab+ 10b –b 2
100 a2 + 100 a + 10b - b2
100 (a) ( a + 1 ) + b(10 - b)
100( a )(a+1)+bd