They are the triangular numbers.
Un = 1 + 2 + 3 + 4 + ... + n = n*(n+1)/2 for all integers n > 0
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it adds one more each time. 1+2=3 3+3=6 6+4=10 10+5=15 the next one would be 15+6=21------------------------------------------------With U{n} = (7n⁵ - 105n⁴ + 595n³ - 1555n² + 1938n - 840)/40U{1, ..., 5} = {1, 3, 6, 10, 15}U{6} = 42However, I suspect the rule your teacher wants is that it is the triangular numbers given by U{n} = ½n(n + 1)
1/10 divided by 6 2/3 = 3/200= 1/10÷62/3=1/10÷20/3=1/10 *3/20= 3/200
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3-6 seconds rule = -3
The difference increases by one. 3, 6, 10, 15, 21, 28, 36 and so on.