STEP II 2007 question 3 solution - The Student Room
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STEP II 2007 question 3 solution

TSR Wiki > Study Help > Subjects and Revision > Mathematics > STEP > STEP II 2007 question 3 solution


Part (a)


x = a\tan(\theta)

This gives:

\frac{dx}{d\theta} = a\sec^2(\theta)

Noting that 1 + \tan^2(\theta) = \sec^2(\theta), substitution gives:

\int_^\frac{d\theta}{a} = \frac{\theta}{a} + constant

So the integral is:

\frac{1}{a}\arctan(\frac{x}{a}) Q.E.D


Part (b)


 I = \int_0^\frac{\Pi}{2}\frac{\cos x}{1+\sin^2x}dx

Make the substitution u = \sin(x) which gives:

\frac{dx}{du} = \frac{1}{\cos x}

I = \int_0^1\frac{1}{1+u^2}du = \arctan(1) - \arctan(0) = \frac{\pi}{4}


Part (c)


As indicated use the following substitution:

 t = \tan(\frac{x}{2}) giving \frac{dt}{dx} = \frac{1}{2}sec^2(\frac{x}{2})

The following identities are used:

\tan^2(\frac{x}{2}) = \frac{1-\cos x}{1 + \cos x} and \sec^2(\frac{x}{2}) = 1 + \tan^2(\frac{x}{2}) = \frac{2}{1+\cos x}

The integral becomes:

\int_0^\frac{\pi}{2}\frac{ \frac{2\cos x}{1+\cos x} }{1 + 6(\frac{1-\cos x}{1+\cos x}) + (\frac{1-\cos x}{1+\cos x})^2 } 0.5\sec^2\frac{x}{2} dx


=2\int_0^\frac{\pi}{2}\frac{ \frac{\cos x}{(1+\cos x)^2} }{1 + 6(\frac{1-\cos x}{1+\cos x}) + (\frac{1-\cos x}{1+\cos x})^2 } dx

=2\int_0^\frac{\pi}{2}\frac{\cos x}{(1+\cos x)^2+6(1-\cos x)(1+\cos x)+(1-\cos x)^2} dx

=2\int_0^\frac{\pi}{2}\frac{\cos x}{2(1+\cos^2 x) + 6\sin^2 x} dx

=0.5 \int_0^\frac{\pi}{2}\frac{\cos x}{1+\sin^2 x} dx

=0.5 I



Part (d)


Use the same method as in part (c) to show that this integral becomes:

0.5\int_0^\frac{\pi}{2}\frac{\cos x}{1+3\sin^2x} dx

= 1/6 \int_0^\frac{\pi}{2}\frac{\cos x}{1/3+\sin^2x} dx

= 1/6 \int_0^1\frac{1}{1/3 + u^2} du

= 1/6 \sqrt{3}\arctan(\sqrt{3})

= 1/6 \sqrt{3} \frac{\pi}{3}

= \frac{\pi}{6 \sqrt{3}}

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