400=(1+x)*x/2

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Solution for 400=(1+x)*x/2 equation: 


x in (-oo:+oo)

400 = (x*(x+1))/2 // - (x*(x+1))/2

400-((x*(x+1))/2) = 0

(-1*x*(x+1))/2+400 = 0

(-1*x*(x+1))/2+(2*400)/2 = 0

2*400-1*x*(x+1) = 0

800-x^2-x = 0

800-x^2-x = 0

800-x^2-x = 0

DELTA = (-1)^2-(-1*4*800)

DELTA = 3201

DELTA > 0

x = (3201^(1/2)+1)/(-1*2) or x = (1-3201^(1/2))/(-1*2)

x = (3201^(1/2)+1)/(-2) or x = (1-3201^(1/2))/(-2)

(x-((3201^(1/2)+1)/(-2)))*(x-((1-3201^(1/2))/(-2))) = 0

((x-((3201^(1/2)+1)/(-2)))*(x-((1-3201^(1/2))/(-2))))/2 = 0

((x-((3201^(1/2)+1)/(-2)))*(x-((1-3201^(1/2))/(-2))))/2 = 0 // * 2

(x-((3201^(1/2)+1)/(-2)))*(x-((1-3201^(1/2))/(-2))) = 0

( x-((1-3201^(1/2))/(-2)) )

x-((1-3201^(1/2))/(-2)) = 0 // + (1-3201^(1/2))/(-2)

x = (1-3201^(1/2))/(-2)

( x-((3201^(1/2)+1)/(-2)) )

x-((3201^(1/2)+1)/(-2)) = 0 // + (3201^(1/2)+1)/(-2)

x = (3201^(1/2)+1)/(-2)

x in { (1-3201^(1/2))/(-2), (3201^(1/2)+1)/(-2) }

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