Q. Find the value of ∫389−x3dx. Express your answer as a constant times ln6.Answer: □ln6
Recognize standard form: Recognize the integral as a standard form.The integral ∫9−x3dx can be recognized as a form of the integral ∫a−x1dx, which is a standard form whose antiderivative is −ln∣a−x∣.
Substitution with u: Rewrite the integral with a substitution.Let u=9−x, then du=−dx. We need to adjust the integral to match this substitution, so we multiply by −1 inside and outside the integral to get −3∫udu.
Change limits of integration: Change the limits of integration.When x=3, u=9−3=6. When x=8, u=9−8=1. So the new limits of integration are from u=6 to u=1.
Perform integration: Perform the integration.The integral −3∫udu from u=6 to u=1 is −3[ln∣u∣]61.
Evaluate definite integral: Evaluate the definite integral.Plugging in the limits of integration, we get −3[ln∣1∣−ln∣6∣]. Since ln∣1∣=0, this simplifies to −3[−ln∣6∣].
Simplify expression: Simplify the expression.The expression −3[−ln∣6∣] simplifies to 3ln∣6∣, which is 3ln6 because the absolute value of a positive number is the number itself.
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