Question

Diketahui rumus fungsi f(x) = ax + b. Jika f(-2) = 2 dan f(3) = -8, nilai f(7) adalah......
A. -21
B. -18
C. -16
D. 8
E. 12

Answer 1

F(x) = ax+b
f(-2) = 2
2 = a(-2)+b
2 = -2a+b .. (1)

f(x) = ax+b
f(3) = -8
-8 = a(3)+b
-8 = 3a+b .. (2)

persamaan 1 dan 2 dieliminasi
(1) .. 2 = -2a+b
(2) .. -8 = 3a+b
kurangkan
10 = -5a
a = -2

ambil persamaan 1
2 = -2a+b
2 = -2(-2)+b
2 = 4+b
b = -2

maka f(x) menjadi
f(x) = -2x-2
f(7) = -2(7)-2
f(7) = -14-2
f(7) = -16 (C)

Answer 2

-2a+b=2
3a+b=7

-5a=10, a=-2
-2x-2+b=2, b=-2

f(7)=7x-2+-2=-16