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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
(30*x^2-(42*x))/x = (-20*x^2-(7*x))/x // - (-20*x^2-(7*x))/x
(30*x^2-(42*x))/x-((-20*x^2-(7*x))/x) = 0
(30*x^2-42*x)/x-((-20*x^2-7*x)/x) = 0
(30*x^2-42*x)/x+(-1*(-20*x^2-7*x))/x = 0
30*x^2-1*(-20*x^2-7*x)-42*x = 0
50*x^2-35*x = 0
50*x^2-35*x = 0
5*x*(10*x-7) = 0
10*x-7 = 0 // + 7
10*x = 7 // : 10
x = 7/10
5*x*(x-7/10) = 0
(5*x*(x-7/10))/x = 0
(5*x*(x-7/10))/x = 0 // * x
5*x*(x-7/10) = 0
( 5*x )
5*x = 0 // : 5
x = 0
( x-7/10 )
x-7/10 = 0 // + 7/10
x = 7/10
x in { 0}
x = 7/10
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