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4/4 Number series

Find the missing number: 1, 3, 6, 10, ?, 21

The seventh number is 28.

The eight number is 36.

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What is ten divided by two?

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JuniperBsays...

The sequence goes: 1, 3, 6, 10, 15, 21, 28, 36..

Series formula: x(n) = n(n+1)/2

Using the formula, the ninth term is 45

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