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X^2-4x-5=0
D=(-4)^2-4*1*(-5)=36
x1,2 = (4+-√36) / 2 = (4+-6) / 2 = 2+-3
x1 = 2-3 = -1,
x2 = 2+3 = 5.
-4x^2-8xy-4y^2=-4(x^2+2xy+y^2)=-4(x+y)^2
X^2 - 11x + 30 = 0
D = 121 - 120 = 1
x1 =( 11 + 1)/2 = 12/2 = 6
x2 = ( 11 - 1)/2 = 10/2 = 5