Diketahui f(x)= x+2 dan g(x)= x pangkat 2 -7x+6. Jika (f•g)(a)=(g•f)(a), nilai a=…

F (x) = x + 2
g (x) = x² – 7x + 6

fog (a) = f (g (a)) 
            = f (a² – 7a + 6)
            = a² – 7a + 6 + 2
            = a² – 7a + 8

gof (a) = g (f(a))
            = g (a + 2)
            = (a + 2)² – 7 (a + 2) + 6
            = a² + 4a + 4 – 7a – 14 + 6
            = a² – 3a – 4

fog (a) = gof (a)
a² – 7a + 8 = a² – 3a – 4
a² – 7a + 8 – a² + 3a + 4 = 0
-4a + 12 = 0
4a = 12
a = 3

Jawaban : C