OCR
FP3
2008
January
Q4
8 marks
Standard +0.8
The integrals \(C\) and \(S\) are defined by
$$C = \int_0^{3\pi} e^{3x} \cos 3x \, dx \quad \text{and} \quad S = \int_0^{3\pi} e^{3x} \sin 3x \, dx.$$
By considering \(C + iS\) as a single integral, show that
$$C = -\frac{1}{3}(2 + 3e^{\pi}),$$
and obtain a similar expression for \(S\). [8]
(You may assume that the standard result for \(\int e^{kx} dx\) remains true when \(k\) is a complex constant, so that \(\int e^{(a+ib)x} dx = \frac{1}{a+ib}e^{(a+ib)x}\).)