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Kelas 8 Matematika
Bab Persamaan Garis

(0, -1) dan (3, 2)

(y - y1)/(y2 - y1) = (x - x1)/(x2 - x1)
(y + 1)/(2 + 1) = (x - 0) / (3 - 0)
(y + 1) / 3 = x/3

** 3 : 3 = 1 **

y + 1 = x
y = x - 1
f(x) = x - 1

f(30) = 30 - 1
f(30) = 29