微积分I(标准路径) · 免费预览

§5.3 Integration by substitution (continued)

我们如何着手为更复杂的代数函数寻找原函数的代数公式?

1. Summary

Summary

2. Antiderivative of tangent

Rewriting tan⁡(x)=sin⁡(x)cos⁡(x)\tan(x) = \frac{\sin(x)}{\cos(x)} and substituting u=cos⁡(x)u = \cos(x) shows that ∫tan⁡(x) dx\int\tan(x)\,dx equals

3. Why sine cubed resists the obvious substitution

For ∫sin⁡3(x) dx\int\sin^3(x)\,dx, the substitution u=sin⁡(x)u = \sin(x) fails because

4. The working substitution for sine cubed

Writing sin⁡3(x)=sin⁡(x)(1−cos⁡2(x))\sin^3(x) = \sin(x)(1 - \cos^2(x)) and using u=cos⁡(x)u = \cos(x) gives ∫sin⁡3(x) dx\int\sin^3(x)\,dx equal to

5. Power consumption through a day

A town consumes power at rate r(t)=4+sin⁡(0.263t+4.7)+cos⁡(0.526t+9.4)r(t) = 4 + \sin(0.263t + 4.7) + \cos(0.526t+9.4) megawatts. The integral ∫024r(t) dt\int_0^{24} r(t)\,dt

6. Sine cubed over a half period

Using the antiderivative 13cos⁡3(x)−cos⁡(x)+C\frac{1}{3}\cos^3(x) - \cos(x) + C, evaluate ∫0π/2sin⁡3(x) dx\int_0^{\pi/2}\sin^3(x)\,dx as a fraction.

7. Root times a shifted variable

Using ∫xx−1 dx=25(x−1)5/2+23(x−1)3/2+C\int x\sqrt{x-1}\,dx = \frac{2}{5}(x-1)^{5/2} + \frac{2}{3}(x-1)^{3/2} + C, evaluate ∫12xx−1 dx\int_1^2 x\sqrt{x-1}\,dx as a fraction.

8. Average power draw

Using ∫024r(t) dt≈95.7809\int_0^{24} r(t)\,dt \approx 95.7809 megawatt hours, compute the average power draw over the day, rounded to two decimals.

9. Antiderivative of cotangent

The integral ∫cot⁡(x) dx\int\cot(x)\,dx equals −ln⁡∣csc⁡(x)∣+C-\ln|\csc(x)| + C.

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