She needs to build a line of dominoes 21 and 1/4 in a long with each domino spaced 1 and 1/4 in. apart. How many will she need?

17 dominoes

Step-by-step explanation:

21 and 1/4 in divided by how far they need to be spaced apart, 1 and 1/4 in, is equal to 17 dominoes.

(n^3 - n)

= n(n-1)(n+1)

since it is divisible by 3 and 2 ,because it is a multiple 3 consecutive number

if, n-1 is odd then n+1 is also odd, then it is a multiple of 9

but if, n-1 is even then n+1 is also even, then it is a multiple of 4

so, largest multiple = max(9*2, 4*3) = max(18, 12) = 18

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