21=(x^2-x)/2

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Solution for 21=(x^2-x)/2 equation: 


x in (-oo:+oo)

21 = (x^2-x)/2 // - (x^2-x)/2

21-((x^2-x)/2) = 0

(-1*(x^2-x))/2+21 = 0

(-1*(x^2-x))/2+(2*21)/2 = 0

2*21-1*(x^2-x) = 0

x-x^2+42 = 0

x-x^2+42 = 0

x-x^2+42 = 0

DELTA = 1^2-(-1*4*42)

DELTA = 169

DELTA > 0

x = (169^(1/2)-1)/(-1*2) or x = (-169^(1/2)-1)/(-1*2)

x = -6 or x = 7

(x+6)*(x-7) = 0

((x+6)*(x-7))/2 = 0

((x+6)*(x-7))/2 = 0 // * 2

(x+6)*(x-7) = 0

( x+6 )

x+6 = 0 // - 6

x = -6

( x-7 )

x-7 = 0 // + 7

x = 7

x in { -6, 7 }

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