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y in (-oo:+oo)
y*((y+1)/2) = 6/7 // - 6/7
y*((y+1)/2)-(6/7) = 0
y*((y+1)/2)-6/7 = 0
(y*(y+1))/2-6/7 = 0
(7*y*(y+1))/(2*7)+(-6*2)/(2*7) = 0
7*y*(y+1)-6*2 = 0
7*y^2+7*y-12 = 0
7*y^2+7*y-12 = 0
7*y^2+7*y-12 = 0
DELTA = 7^2-(-12*4*7)
DELTA = 385
DELTA > 0
y = (385^(1/2)-7)/(2*7) or y = (-385^(1/2)-7)/(2*7)
y = (385^(1/2)-7)/14 or y = (-(385^(1/2)+7))/14
(y+(385^(1/2)+7)/14)*(y-((385^(1/2)-7)/14)) = 0
((y+(385^(1/2)+7)/14)*(y-((385^(1/2)-7)/14)))/(2*7) = 0
((y+(385^(1/2)+7)/14)*(y-((385^(1/2)-7)/14)))/(2*7) = 0 // * 2*7
(y+(385^(1/2)+7)/14)*(y-((385^(1/2)-7)/14)) = 0
( y+(385^(1/2)+7)/14 )
y+(385^(1/2)+7)/14 = 0 // - (385^(1/2)+7)/14
y = -((385^(1/2)+7)/14)
( y-((385^(1/2)-7)/14) )
y-((385^(1/2)-7)/14) = 0 // + (385^(1/2)-7)/14
y = (385^(1/2)-7)/14
y in { -((385^(1/2)+7)/14), (385^(1/2)-7)/14 }
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