Question 168742
Just from thinking about it:
4*5 = 20, and -4 x -5 = 20
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The hard way:
n*(n+1)=20
{{{n^2 + n - 20 = 0}}}
*[invoke solve_quadratic_equation 1,1,-20]
If n = 4, n+1 is 5.
If n = -5, n+1 is -4.