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1.
560
The smallest tetrahedral number that is the product of two tetrahedral numbers both greater than one. Also the second smallest number and smallest tetrahedral number divisible by 1, 4, 10, 20, 35, and 56 (the first six tetrahedral numbers; the smallest number divisible by all those is 280)
tetra(3) * tetra(6) = (3*4*5/6)*(6*7*8/6) = (3*4*5/6)*(7*8) = (60/6)*56 = 10*56 = 560 = tetra(14). 560/4=140, 560/10= 56, 560/20= 28, 560/35= 16, 560/56= 10 (all quotients are even so 280, half of 560, is divisible by all divisors given (1, 4, 10, 20, 35, and 56)).
by EmeraldofSeattle March 01, 2007