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D( a )
a+20 = 0
a+20 = 0
a+20 = 0
a+20 = 0 // - 20
a = -20
a in (-oo:-20) U (-20:+oo)
100000 = 3000/(a+20) // - 3000/(a+20)
100000-(3000/(a+20)) = 0
100000-3000*(a+20)^-1 = 0
100000-3000/(a+20) = 0
(100000*(a+20))/(a+20)-3000/(a+20) = 0
100000*(a+20)-3000 = 0
100000*a+1997000 = 0
(100000*a+1997000)/(a+20) = 0
(100000*a+1997000)/(a+20) = 0 // * a+20
100000*a+1997000 = 0
100000*a+1997000 = 0 // - 1997000
100000*a = -1997000 // : 100000
a = -1997000/100000
a = -1997/100
a = -1997/100
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