Q. Find the value of ∫684−x4dx. Express your answer as a constant times ln2.Answer: □ln2
Recognize standard form: Recognize the integral as a standard form.The integral ∫4−x4dx can be recognized as a standard form of the integral ∫B−xAdx, which can be solved by using the substitution method.
Use substitution to simplify: Use substitution to simplify the integral.Let u=4−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 −∫u−4du.
Rewrite in terms of u: Rewrite the integral in terms of u.The integral now becomes −4∫u1du, which is a standard form for the natural logarithm function.
Integrate with respect to u: Integrate with respect to u.The integral of u1 with respect to u is ln∣u∣, so the integral becomes −4ln∣u∣+C, where C is the constant of integration. However, since we are evaluating a definite integral, we do not need to include the constant of integration.
Substitute back in x: Substitute back in terms of x.We originally let u=4−x, so we substitute back to get −4ln∣4−x∣.
Evaluate definite integral: Evaluate the definite integral from 6 to 8.We need to evaluate −4ln∣4−x∣ from 6 to 8. This gives us −4[ln∣4−8∣−ln∣4−6∣].
Simplify expression: Simplify the expression.We have −4[ln∣−4∣−ln∣−2∣], which simplifies to −4[ln(4)−ln(2)].
Combine logarithms: Use properties of logarithms to combine the terms.We can use the property ln(a)−ln(b)=ln(ba) to combine the logarithms: −4ln(24).
Simplify logarithm: Simplify the logarithm.Since 24=2, we have −4ln(2).
Express as constant times ln(2): Express the answer as a constant times ln(2).The final answer is −4ln(2), which is the value of the definite integral from 6 to 8.
More problems from Find indefinite integrals using the substitution and by parts