If f (x) = AX2 + BX, and 1 ≤ f (1) ≤ 2,2 ≤ f (1) ≤ 4, find the range of F (2)

If f (x) = AX2 + BX, and 1 ≤ f (1) ≤ 2,2 ≤ f (1) ≤ 4, find the range of F (2)

From F (x) = ax ^ 2 + BX, we can get f (- 1) = A-B, ① f (1) = a + B, ② f (- 2) = 4a-2b, from ① + ② we can get a = [f (1) + F (- 1)], from ② - ① we can get b = [f (1) - f (- 1)], thus f (- 2) = 2 [f (1) + F (- 1)] - [f (1) - f (- 1)] = 3f (- 1) + F (1) and 1 ≤ f (1) ≤ 2,2