tentukan fungsi kuadrat yg grafiknya melalui titik kordinat (0,5),(1,6),(-1,12)

jawab
fungsi kuadrat y = ax² + bx + c
(0,5) → 5 = a(0)² +b(0) + c  →  c = 5
(-1, 12) →12 = a - b + c    → a - b + 5 = 12 → a - b = 7 ...(i)
(1, 6) → 6 = a + b + c  → a + b + 5 = 6 →  a + b  = 1 ...(ii)

(i) dan (ii)
a - b = 7
a + b = 1 |(+)
2a = 8  → a = 4
a - b = 7 → b = a- 7 → b = 4-7 = -3

(a,b,c) = (4, -3, 5)
f(x) = ax²  + bx + c → f(x) =  4x² - 3x + 5