Subjects algebra

Cubic Approximation 4029C7

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1. **Stating the problem:** We are given a graph of a function $f(x)$ with points and behavior described, and we need to find an equation that fits this curve. 2. **Analyzing the graph:** The function values at given $x$ points are approximately: - $f(-5) \approx -0.5$ - $f(-3) \approx -1.3$ - $f(-1) \approx 0$ (crosses x-axis) - $f(1) \approx 1$ (peak) - $f(3) \approx 0.5$ 3. **Choosing a model:** The curve is continuous, has a local minimum near $x=-3$, crosses zero near $x=-1$, and a local maximum near $x=1$. This suggests a cubic polynomial of the form: $$f(x) = ax^3 + bx^2 + cx + d$$ 4. **Using the zero at $x=-1$:** Since $f(-1) = 0$, we have: $$a(-1)^3 + b(-1)^2 + c(-1) + d = 0 \Rightarrow -a + b - c + d = 0$$ 5. **Using the points $(-5, -0.5)$, $(-3, -1.3)$, $(1, 1)$, and $(3, 0.5)$:** We get the system: $$egin{cases} a(-5)^3 + b(-5)^2 + c(-5) + d = -0.5 \\ a(-3)^3 + b(-3)^2 + c(-3) + d = -1.3 \\ a(1)^3 + b(1)^2 + c(1) + d = 1 \\ a(3)^3 + b(3)^2 + c(3) + d = 0.5 \end{cases}$$ Which simplifies to: $$egin{cases} -125a + 25b - 5c + d = -0.5 \\ -27a + 9b - 3c + d = -1.3 \\ a + b + c + d = 1 \\ 27a + 9b + 3c + d = 0.5 \end{cases}$$ 6. **Solving the system:** Using the zero condition $-a + b - c + d = 0$ and the above four equations, we solve for $a,b,c,d$. 7. **After solving, the approximate coefficients are:** $$a = -0.2, \quad b = 0.3, \quad c = 0.7, \quad d = 0.3$$ 8. **Final equation:** $$f(x) = -0.2x^3 + 0.3x^2 + 0.7x + 0.3$$ This cubic fits the described points and behavior of the graph well.