Gọi năng suất dự kiến là`: a `
ta có :
`(120)/a = 2 + ( 120 - 2a )/( a + 3 ) + 8/5`
`⇔(600)/a=18+(600-10a)/(a+3)`
`⇔600=18a+(600a-10a^2)/(a+3)`
`⇔600(a+3)=18a(a+3)+600a-10a^2`
`⇔600a+1800=18a^2+54a+600-10a^2`
`⇔8a^2 + 54a - 1800 = 0 `
`⇔(a-12)(a+18,75)=0`
`⇔a-12=0`
`⇔a=12`