Porisma

Worked solution

Ten direct integrals and reverse chain rules

Find the following integrals:

(a) ∫x(x+3) dx\displaystyle\int \sqrt{x}(x+3)\,dx

(b) ∫(y+1y)2 dy\displaystyle\int \left(\sqrt y+\dfrac1{\sqrt y}\right)^2\,dy

(c) ∫x+1(x2+2x+3)3 dx\displaystyle\int \dfrac{x+1}{(x^2+2x+3)^3}\,dx

(d) ∫ln⁡xx dx\displaystyle\int \dfrac{\ln x}x\,dx

(e) ∫3t1−t dt\displaystyle\int \dfrac{3t}{1-t}\,dt

(f) ∫1xln⁡x dx\displaystyle\int \dfrac1{x\ln x}\,dx

(g) ∫ex1+ex dx\displaystyle\int \dfrac{e^x}{1+e^x}\,dx

(h) ∫exx dx\displaystyle\int \dfrac{e^{\sqrt x}}{\sqrt x}\,dx

(i) ∫e1/xx2 dx\displaystyle\int \dfrac{e^{1/x}}{x^2}\,dx

(j) ∫20172017x dx\displaystyle\int 2017^{2017x}\,dx.

CJC · Discussion · Question 3

Answer

(a) 25x5/2+2x3/2+C\dfrac25x^{5/2}+2x^{3/2}+C.

(b) y22+2y+ln⁡∣y∣+C\dfrac{y^2}2+2y+\ln|y|+C.

(c) −14(x2+2x+3)2+C-\dfrac1{4(x^2+2x+3)^2}+C.

(d) (ln⁡x)22+C\dfrac{(\ln x)^2}2+C.

(e) −3t−3ln⁡∣1−t∣+C-3t-3\ln|1-t|+C.

(f) ln⁡∣ln⁡x∣+C\ln|\ln x|+C.

(g) ln⁡(1+ex)+C\ln(1+e^x)+C.

(h) 2ex+C2e^{\sqrt x}+C.

(i) −e1/x+C-e^{1/x}+C.

(j) 20172017x2017ln⁡2017+C\dfrac{2017^{2017x}}{2017\ln2017}+C.

(a)

Expand the integrand as x3/2+3x1/2x^{3/2}+3x^{1/2} and integrate each power: x5/2/(5/2)+3x3/2/(3/2)x^{5/2}/(5/2)+3x^{3/2}/(3/2). The square root restricts the original real input to x≥0x\geq0.

The integral is 25x5/2+2x3/2+C\dfrac25x^{5/2}+2x^{3/2}+C.

How did this part go?

(b)

The square expands to y+2+1/yy+2+1/y. Integrate term by term. The original reciprocal square root requires y>0y>0, so the printed ln⁡∣y∣\ln|y| is also ln⁡y\ln y on that domain.

The integral is y22+2y+ln⁡∣y∣+C\dfrac{y^2}2+2y+\ln|y|+C.

How did this part go?

(c)

Insert the inner derivative: 12∫2(x+1)(x2+2x+3)−3 dx\dfrac12\int2(x+1)(x^2+2x+3)^{-3}\,dx. The reverse chain rule gives 12(−2)(x2+2x+3)−2\dfrac1{2(-2)}(x^2+2x+3)^{-2}.

The integral is −14(x2+2x+3)2+C-\dfrac1{4(x^2+2x+3)^2}+C.

How did this part go?

(d)

View this as ∫(1/x)(ln⁡x) dx\int(1/x)(\ln x)\,dx and use the derivative of ln x. The real input requires x>0.

The integral is (ln⁡x)22+C\dfrac{(\ln x)^2}2+C.

How did this part go?

(e)

Rewrite 3t/(1−t)=3(−1+1/(1−t))3t/(1-t)=3(-1+1/(1-t)). Integrate the constant and account for the inner derivative −1 in the logarithmic term; t≠1.

The integral is −3t−3ln⁡∣1−t∣+C-3t-3\ln|1-t|+C.

How did this part go?

(f)

Rewrite the numerator as (1/x) dx(1/x)\,dx, the differential of ln x. Apply the logarithmic rule to ln x; the original domain is x>0, x≠1.

The integral is ln⁡∣ln⁡x∣+C\ln|\ln x|+C.

How did this part go?

(g)

The numerator differentiates the denominator. The source first uses ln⁡∣1+ex∣\ln|1+e^x| and then removes the absolute value because eˣ>0 for every real x.

The integral is ln⁡(1+ex)+C\ln(1+e^x)+C.

How did this part go?

(h)

Write the integral as 2∫12xex dx2\int\dfrac1{2\sqrt x}e^{\sqrt x}\,dx. The reciprocal factor is the derivative of √x; x>0.

The integral is 2ex+C2e^{\sqrt x}+C.

How did this part go?

(i)

Write −∫(−1/x2)e1/x dx-\int(-1/x^2)e^{1/x}\,dx and use the derivative of 1/x. Retain x≠0.

The integral is −e1/x+C-e^{1/x}+C.

How did this part go?

(j)

For base 2017 the derivative includes both ln2017 and the inner derivative 2017. Divide by both factors.

The integral is 20172017x2017ln⁡2017+C\dfrac{2017^{2017x}}{2017\ln2017}+C.

How did this part go?

Common mistakes

Retain absolute values in logarithms unless the original domain makes the argument positive.

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