what is the tricks for multiply 99?
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10b + c:
bc= 10b + c = 10(a - 1) + (9 - (a - 1)) = 10a - 10 + 10 - a = 9a,
as required. Similarly, for a 2-digit a:
bc= 100b + c = 100(a - 1) + (99 - (a - 1)) = 100a - 100 + 100 - a = 99a
T H A N K S
B Y E
bc= 10b + c = 10(a - 1) + (9 - (a - 1)) = 10a - 10 + 10 - a = 9a,
as required. Similarly, for a 2-digit a:
bc= 100b + c = 100(a - 1) + (99 - (a - 1)) = 100a - 100 + 100 - a = 99a
T H A N K S
B Y E
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