If it's not what You are looking for type in the equation solver your own equation and let us solve it.
D( x )
400-(20*x) = 0
400-(20*x) = 0
400-(20*x) = 0
400-20*x = 0
400-20*x = 0 // - 400
-20*x = -400 // : -20
x = -400/(-20)
x = 20
x in (-oo:20) U (20:+oo)
x = 5/(400-(20*x)) // - 5/(400-(20*x))
x-(5/(400-(20*x))) = 0
x-5*(400-20*x)^-1 = 0
x-5/(400-20*x) = 0
(x*(400-20*x))/(400-20*x)-5/(400-20*x) = 0
x*(400-20*x)-5 = 0
400*x-20*x^2-5 = 0
400*x-20*x^2-5 = 0
5*(80*x-4*x^2-1) = 0
80*x-4*x^2-1 = 0
DELTA = 80^2-(-4*(-1)*4)
DELTA = 6384
DELTA > 0
x = (6384^(1/2)-80)/(-4*2) or x = (-6384^(1/2)-80)/(-4*2)
x = (4*399^(1/2)-80)/(-8) or x = (-4*399^(1/2)-80)/(-8)
5*(x-((4*399^(1/2)-80)/(-8)))*(x-((-4*399^(1/2)-80)/(-8))) = 0
(5*(x-((4*399^(1/2)-80)/(-8)))*(x-((-4*399^(1/2)-80)/(-8))))/(400-20*x) = 0
(5*(x-((4*399^(1/2)-80)/(-8)))*(x-((-4*399^(1/2)-80)/(-8))))/(400-20*x) = 0 // * 400-20*x
5*(x-((4*399^(1/2)-80)/(-8)))*(x-((-4*399^(1/2)-80)/(-8))) = 0
( 5 )
5 = 0
x belongs to the empty set
( x-((-4*399^(1/2)-80)/(-8)) )
x-((-4*399^(1/2)-80)/(-8)) = 0 // + (-4*399^(1/2)-80)/(-8)
x = (-4*399^(1/2)-80)/(-8)
( x-((4*399^(1/2)-80)/(-8)) )
x-((4*399^(1/2)-80)/(-8)) = 0 // + (4*399^(1/2)-80)/(-8)
x = (4*399^(1/2)-80)/(-8)
x in { (-4*399^(1/2)-80)/(-8), (4*399^(1/2)-80)/(-8) }
| 14.6+14.6= | | -8(10-x)=8 | | Y=-0.1(5)x | | 2x(x-5)=5x-1 | | 4(x-33)+7(x+1)=6(x-3)+7 | | 12=-3x-3+6x | | 5(3x-2)+4=2(5x-1)+1 | | (e^4x)-4x=y | | y=(e^4x)+4x | | 6t+15=-7t+80 | | 4c+15=9c+65 | | 2c+9=8c+63 | | -8q-5=-5q+19 | | -5f+1=-7f+27 | | 4a+4=-a+24 | | -7j-2=-4j+1 | | 6(x-1)-4(x+2)=4 | | 19=9p-7-3p | | 3w-1=-2w-1 | | -2f+3=-f+4 | | 5x-8(13y)=0 | | 2.3-5.4v=15.26 | | 5(3d+4)=2(4d-4) | | 2x-5x=-3x | | 2X^2-3h-4= | | 3msquare+15m=1 | | Z^3-(1-i)z^2+z-1+i=0 | | 3x^4-8x^2=0 | | 11x-37= | | 6x^2-18x+10=0 | | 2/3(15-9x)-3/4(12-8x) | | -4(x-2)+14=x+37 |