Di ketahui f (4-3x)=6x+5. tentukan nilai f(8)+f(-4)

Di ketahui f (4-3x)=6x+5. tentukan nilai f(8)+f(-4)

Jawaban : 18

⚠️INGAT!
Fungsi
Jika diketahui f(a + bx) = px, maka :
▪️dimisalkan,
u = a + bx
↔️ bx = a – u
↔️ x = (a – u)/b
Diperoleh :
f(u) = p[(a – u)/b] f(u) = p(a – u)/b
▪️saat u = x, maka f(x) = p(a – x)/b

Sehingga :
f(4 – 3x) = 6x + 5

👇
u = 4 – 3x
↔️ 3x = 4 – u
↔️ x = (4 – u)/3

👇
f(u) = 6[(4 – u)/3] + 5
f(u) = 2(4 – u) + 5
f(u) = 8 – 2u + 5
f(u) = 13 – 2u

👇
u = x, maka f(x) = 13 – 2x

▪️f(8)
= 13 – 2(8)
= 13 – 16
= -3

▪️f(-4)
= 13 – 2(-4)
= 13 – (-8)
= 13 + 8
= 21

👇
f(8) + f(-4)
= -3 + 21
= 18

Jadi, jawaban yang benar adalah 18