`c) M = (a^2 + a + 1)/(a - 1)`
`=> M. (a - 1) = (a^2 + a + 1)/(a - 1). (a - 1)`
`= a^2 + a + 1`
`= a^2 + 1/2a + 1/2a + 1/4 + 3/4`
`= a^2 + 1/2a + 1/2a + 1/2. 1/2 + 3/4`
`= (a^2 + 1/2a) + (1/2a + 1/2. 1/2) + 3/4`
`= a. (a + 1/2) + 1/2(a + 1/2) + 3/4`
`= (a + 1/2)^2 + 3/4`
Vì `(a + 1/2)^2 >= 0 ∀ x ∈ R`
`=> (a + 1/2)^2 >= 3/4 ∀ x ∈ R`
`=> M. (a - 1) >= 3/4 ∀ x ∈ R`
`⇒ Mi n M. (a - 1) = 3/4 <=> a + 1/2 = 0`
`<=> a = -1/2`
Vậy `Mi n M. (a - 1) = 3/4 <=> a = -1/2`
