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Worked solution

Complete the square and match a standard integral

Find the following integrals:

(a) ∫12−9x2 dx\displaystyle\int \dfrac1{2-9x^2}\,dx

(b) ∫16+4y−y2 dy\displaystyle\int \dfrac1{6+4y-y^2}\,dy

(c) ∫1y2−4y−13 dy\displaystyle\int \dfrac1{y^2-4y-13}\,dy

(d) ∫12y2−4y−1 dy\displaystyle\int \dfrac1{2y^2-4y-1}\,dy

(e) ∫1x2+4x+20 dx\displaystyle\int \dfrac1{x^2+4x+20}\,dx

(f) ∫12x2+3x+10 dx\displaystyle\int \dfrac1{2x^2+3x+10}\,dx

(g) ∫15−4y−y2 dy\displaystyle\int \dfrac1{\sqrt{5-4y-y^2}}\,dy

(h) ∫125−9x2 dx\displaystyle\int \dfrac1{\sqrt{25-9x^2}}\,dx.

CJC · Discussion · Question 4

Answer

(a) 162ln⁡∣2+3x2−3x∣+C\dfrac1{6\sqrt2}\ln\left|\dfrac{\sqrt2+3x}{\sqrt2-3x}\right|+C.

(b) 1210ln⁡∣10+y−210−y+2∣+C\dfrac1{2\sqrt{10}}\ln\left|\dfrac{\sqrt{10}+y-2}{\sqrt{10}-y+2}\right|+C.

(c) 1217ln⁡∣y−2−17y−2+17∣+C\dfrac1{2\sqrt{17}}\ln\left|\dfrac{y-2-\sqrt{17}}{y-2+\sqrt{17}}\right|+C.

(d) 612ln⁡∣2y−2−32y−2+3∣+C\dfrac{\sqrt6}{12}\ln\left|\dfrac{\sqrt2y-\sqrt2-\sqrt3}{\sqrt2y-\sqrt2+\sqrt3}\right|+C.

(e) 14tan⁡−1(x+24)+C\dfrac14\tan^{-1}\left(\dfrac{x+2}4\right)+C.

(f) 271tan⁡−1(4x+371)+C\dfrac2{\sqrt{71}}\tan^{-1}\left(\dfrac{4x+3}{\sqrt{71}}\right)+C.

(g) sin⁡−1(y+23)+C\sin^{-1}\left(\dfrac{y+2}3\right)+C.

(h) 13sin⁡−1(3x5)+C\dfrac13\sin^{-1}\left(\dfrac{3x}5\right)+C.

(a)

Factor out 9: 19∫12/9−x2 dx\dfrac19\int\dfrac1{2/9-x^2}\,dx. Apply the a2−x2a^2-x^2 logarithmic formula with a=2/3a=\sqrt2/3. The coefficient is (1/9)/(2a)=1/(62)(1/9)/(2a)=1/(6\sqrt2).

The integral is 162ln⁡∣2+3x2−3x∣+C\dfrac1{6\sqrt2}\ln\left|\dfrac{\sqrt2+3x}{\sqrt2-3x}\right|+C.

How did this part go?

(b)

Complete the square: 6+4y−y2=6−[(y−2)2−4]=10−(y−2)26+4y-y^2=6-[(y-2)^2-4]=10-(y-2)^2. Apply the difference-of-squares logarithmic form.

The integral is 1210ln⁡∣10+y−210−y+2∣+C\dfrac1{2\sqrt{10}}\ln\left|\dfrac{\sqrt{10}+y-2}{\sqrt{10}-y+2}\right|+C.

How did this part go?

(c)

The denominator is (y−2)2−17(y-2)^2-17. Use the (y−2)2−a2(y-2)^2-a^2 form with a=17a=\sqrt{17}, retaining absolute values.

The integral is 1217ln⁡∣y−2−17y−2+17∣+C\dfrac1{2\sqrt{17}}\ln\left|\dfrac{y-2-\sqrt{17}}{y-2+\sqrt{17}}\right|+C.

How did this part go?

(d)

Factor 2 and complete the square: 2y2−4y−1=2[(y−1)2−3/2]2y^2-4y-1=2[(y-1)^2-3/2]. Integrate as 12∫[(y−1)2−3/2]−1 dy\dfrac12\int[(y-1)^2-3/2]^{-1}\,dy. The logarithm coefficient is 12⋅123/2=6/12\dfrac12\cdot\dfrac1{2\sqrt{3/2}}=\sqrt6/12. Multiply numerator and denominator of its ratio by √2 to get the printed form.

The integral is 612ln⁡∣2y−2−32y−2+3∣+C\dfrac{\sqrt6}{12}\ln\left|\dfrac{\sqrt2y-\sqrt2-\sqrt3}{\sqrt2y-\sqrt2+\sqrt3}\right|+C.

How did this part go?

(e)

Complete the square to (x+2)2+16(x+2)^2+16. Apply the u2+a2u^2+a^2 arctangent form with u=x+2 and a=4.

The integral is 14tan⁡−1(x+24)+C\dfrac14\tan^{-1}\left(\dfrac{x+2}4\right)+C.

How did this part go?

(f)

Complete the square: 2x2+3x+10=2(x+3/4)2+71/82x^2+3x+10=2(x+3/4)^2+71/8. The printed formula box matches f(x)=2(x+3/4)f(x)=\sqrt2(x+3/4), f′(x)=2f^\prime(x)=\sqrt2 and a2=71/8a^2=71/8. Multiply and divide the integral by √2 to match ∫f′(x)/(f(x)2+a2) dx=(1/a)tan⁡−1(f(x)/a)+C\int f^\prime(x)/(f(x)^2+a^2)\,dx=(1/a)\tan^{-1}(f(x)/a)+C. Simplification gives the stated coefficient 2/√71 and argument (4x+3)/√71.

The integral is 271tan⁡−1(4x+371)+C\dfrac2{\sqrt{71}}\tan^{-1}\left(\dfrac{4x+3}{\sqrt{71}}\right)+C.

How did this part go?

(g)

The radicand is 5−[(y+2)2−4]=9−(y+2)25-[(y+2)^2-4]=9-(y+2)^2. Apply the inverse-sine form. The original integrand requires −5<y<1.

The integral is sin⁡−1(y+23)+C\sin^{-1}\left(\dfrac{y+2}3\right)+C.

How did this part go?

(h)

Factor √9 from the denominator to obtain 19∫[25/9−x2]−1/2 dx\dfrac1{\sqrt9}\int[25/9-x^2]^{-1/2}\,dx. The result is (1/3)sin⁡−1(x/(5/3))(1/3)\sin^{-1}(x/(5/3)), with −5/3<x<5/3.

The integral is 13sin⁡−1(3x5)+C\dfrac13\sin^{-1}\left(\dfrac{3x}5\right)+C.

How did this part go?

Common mistakes

A square-root denominator leads to an inverse-sine form; the same quadratic without a square root can require a logarithm.

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