For positive integers a, B, a △ B is equal to the sum of B consecutive positive integers starting from a, such as 2 △ 3 = 2 + 3 + 4 = 9, or 5 △ 4 = 5 + 6 + 7 + 8 = 26, if 1 △ χ = 15

1 △ χ is the sum of consecutive positive integers starting from 1
1,2,3,..., χ is an arithmetic sequence
χ(1+χ）/2=15
It's time to tidy up
χ^2+χ-30=0
(χ+6)(χ-5)=0
χ = 5 or χ = - 6 (rounding off)
χ=5

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