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D( t )
-2*t-40 = 0
-2*t-40 = 0
-2*t-40 = 0
-2*t-40 = 0 // + 40
-2*t = 40 // : -2
t = 40/(-2)
t = -20
t in (-oo:-20) U (-20:+oo)
t = (-40*2*t)/(-2*t-40) // - (-40*2*t)/(-2*t-40)
t-((-40*2*t)/(-2*t-40)) = 0
80*t*(-2*t-40)^-1+t = 0
(80*t)/(-2*t-40)+t = 0
(80*t)/(-2*t-40)+(t*(-2*t-40))/(-2*t-40) = 0
t*(-2*t-40)+80*t = 0
40*t-2*t^2 = 0
40*t-2*t^2 = 0
2*t*(20-t) = 0
20-t = 0 // - 20
-t = -20 // * -1
t = 20
2*t*(t-20) = 0
(2*t*(t-20))/(-2*t-40) = 0
(2*t*(t-20))/(-2*t-40) = 0 // * -2*t-40
2*t*(t-20) = 0
( 2*t )
2*t = 0 // : 2
t = 0
( t-20 )
t-20 = 0 // + 20
t = 20
t in { 0, 20 }
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