Given: ๐‘“(๐‘ฅ) = ๐‘Ž๐‘ฅ^3 + ๐‘๐‘ฅ^2 โˆ’ 3 and ๐‘“"(โˆ’2) = 0. If it is further given that the point (โˆ’3; 6) lies on the graph of ๐‘“. Show that ๐‘Ž = 1/3 and ๐‘ = 2.

We start off by finding the first derivative of equation f(x) = ax3 + bx2 - 3: f'(x) = 3ax2 + 2bx. We now take the second derivative of equation f, because we have been told that f"(-2) = 0: f"(x) = 6ax + 2b (1). We know that with an x value of -2, equation (1) is equal to 0: f"(-2) = 6a(-2) + 2b = 0-12a +2b = 0 (2). This equation will be used later to find the final answer. We also know that the pointย (โˆ’3; 6) lies on the graph of ๐‘“. Therefore, for an x value of -3, f(x) equals 6: f(-3) = a(-3)2 + b(-3)2 - 3 = 6-27a + 9b = 9 (3). We then solve equations (2) and (3) simultaneously, as we have two unknowns and two equations, and reach the following answer: a = 1/3 ; b = 2. This question would be worth a total of 6 points.

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Answered by Neil L. โ€ข Maths tutor

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