Question

find n if 1+2+3+...+n = 210​

Answer 1

Step-by-step explanation:

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Answer 2

it is an Arithmetic progression

a=1,d=1,s=210

s = n(2a+(n-1)d)/2

210 = n(2×1+(n-1)×1)/2

210 = n(n+1)/2

105 = n(n+1)

105 = n²+n

n²+n-105 = 0