Q. Find a power series for f(x)=8−x4 centered at 2 and determine the interval of convergence. (6
Rewrite Function: To find a power series for f(x)=8−x4 centered at 2, we first rewrite the function in a form that allows us to use the geometric series formula. The geometric series formula is 1−r1=1+r+r2+r3+..., which converges when |r| < 1. We need to express f(x) in a similar form.
Express as Geometric Series: We rewrite f(x) as f(x)=8−x4=6−(x−2)4. This puts the function in a form where we can identify the 'r' term for the geometric series as 6(x−2).
Expand into Power Series: Now we can express f(x) as a geometric series: f(x)=6−(x−2)4=4⋅6⋅(1−6x−2)1=32⋅1−6x−21.
Simplify Power Series: Using the geometric series formula, we expand f(x) into a power series: f(x)=32×(1+6(x−2)+(6(x−2))2+(6(x−2))3+…).
Final Power Series: To simplify the power series, we distribute the 32 across the series: f(x)=32+9(x−2)+54(x−2)2+324(x−2)3+…
Interval of Convergence: The power series for f(x) is now expressed as: f(x)=∑n=0∞3n+1(x−2)n.
Check Endpoints: Next, we need to determine the interval of convergence for the power series. The series converges when |\frac{x-2}{6}| < 1, which means -1 < \frac{x-2}{6} < 1.
Check Endpoints: Next, we need to determine the interval of convergence for the power series. The series converges when |\frac{x-2}{6}| < 1, which means -1 < \frac{x-2}{6} < 1.Solving the inequality for x gives us -6 < x-2 < 6, which simplifies to -4 < x < 8.
Check Endpoints: Next, we need to determine the interval of convergence for the power series. The series converges when |\frac{x-2}{6}| < 1, which means -1 < \frac{x-2}{6} < 1.Solving the inequality for x gives us -6 < x-2 < 6, which simplifies to -4 < x < 8.We must check the endpoints x=−4 and x=8 to see if the series converges at these points. For x=−4, the series becomes 8−(−4)4=124=31, which is a finite number. For x=8, the series is undefined because it would involve division by zero. Therefore, the interval of convergence is -1 < \frac{x-2}{6} < 10.
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