Let f(x) be a quadratic ploynomial in x such that f(x) ≥ 0 for all real numbers x. If f(2) = 0 and f(4) = 6, then f(-2) is equal to
Explanation:
Let f(x) = ax2 + bx + c Since, f(x) ≥ 0, and f(2) = 0, it means the graph of the quadratic lies above x-axis and touches x-axis at x = 2.
Since x = 2 is the only root of the equation (i.e., graph is symmetric about x = 2), ⇒ f(2 + a) = f(2 – a) ⇒ f(2 + 2) = f(2 - 2) ⇒ f(4) = f(0) ⇒ f(0) = 6 ⇒ c = 6
f(2) = 0 ⇒ 4a + 2b + c = 0 ⇒ 4a + 2b = -6 …(2)
f(4) = 6 ⇒ 16a + 4b + c = 6 ⇒ 16a + 4b = 0 …(3)
Solving (2) and (4), we get a = -3/2 and b = -6
⇒ f(-2) = 32(-2)2 – 6 × - 2 + 6 = 6 + 12 + 6 = 24
Hence, option (a).
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