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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
1.96 = (x-65)/x // - (x-65)/x
1.96-((x-65)/x) = 0
(-1*(x-65))/x+1.96 = 0
(-1*(x-65))/x+(1.96*x)/x = 0
1.96*x-1*(x-65) = 0
0.96*x+65 = 0
(0.96*x+65)/x = 0
(0.96*x+65)/x = 0 // * x
0.96*x+65 = 0
0.96*x+65 = 0 // - 65
0.96*x = -65 // : 0.96
x = -65/0.96
x = -67.70833333
x = -67.70833333
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