Question

what is the solution of the equation n(n+1)/2=21​

Answer 1

hope it helps

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Answer 2

Answer:

we get this quadratic equation

$n {}^{2} + n - 42 = 0$

${n}^{2} + 7n - 6n - 42 = 0 \\ n(n + 7) - 6(n + 7) = 0 \\( n - 6)(n + 7) = 0 \\ (n - 6) = 0or(n + 7) = 0 \\n = 6 \: or \: \: n = - 7$