Question #213360

Find the Fourier sine and Fourier cosine integral for the following function 

f(x) = (x2, 0 < x ≤ 10

0, x > 10.


1
Expert's answer
2021-07-05T04:54:31-0400

(i)Fourier sine transform off(x)fs(ω)=2πf(x)sin(ωx)dx=2π010x2sin(ωx)dx+2π100×sin(ωx)dx=1ω2π010x2d(cos(ωx))=2πω2(x2cos(ωx)010+2010xcos(ωx)dx)=2πω2(100cos(100ω)+2ω010xd(sin(ωx)))=2πω2(100cos(100ω)+2ωxsin(ωx)0102ω010sin(ωx)dx)=2πω2(100cos(100ω)+2ωxsin(ωx)010+2ω2cos(ωx)010)=2πω2(100cos(100ω)+20ωsin(20ω)+2ω2(cos(10ω)1))Fourier cosine transform off(x)fc(ω)=2πf(x)cos(ωx)dx=2π010x2cos(ωx)dx+2π100×cos(ωx)dx=1ω2π010x2d(sin(ωx))=2πω2(x2sin(ωx)0102010xsin(ωx)dx)=2πω2(100sin(100ω)+2ω010xd(cos(ωx)))=2πω2(100sin(100ω)+2ωxcos(ωx)0102ω010cos(ωx)dx)=2πω2(100sin(100ω)+2ωxcos(ωx)0102ω2sin(ωx)010)=2πω2(100sin(100ω)+20ωcos(20ω)2ω2sin(10ω))(ii)Fourier sine transform off(x)fc(ω)=2πf(x)cos(ωx)dx=2π01xcos(ωx)dx+2π12(x+1)cos(ωx)dx+2π20×cos(ωx)dx=1ω2π(01xd(sin(ωx))+12(x+1)d(sin(ωx)))=1ω2π(xsin(ωx)0101sin(ωx)dx+(x+1)sin(ωx)1212sin(ωx)dx)=1ω2π(sin(ω)+cos(ωx)ω01+3sin(2ω)2sin(ω)+cos(ωx)ω12)=1ω2π(sin(ω)+cos(ω)ω1ω+3sin(2ω)2sin(ω)+cos(2ω)ωcos(ω))=1ω2π(sin(ω)+3sin(2ω)+cos(2ω)ω1ω)Fourier cosine transform off(x)fc(ω)=2πf(x)cos(ωx)dx=2π01xcos(ωx)dx+2π12(x+1)cos(ωx)dx+2π20×cos(ωx)dx=1ω2π(01xd(sin(ωx))+12(x+1)d(sin(ωx)))=1ω2π(xsin(ωx)0101sin(ωx)dx+(x+1)sin(ωx)1212sin(ωx)dx)=1ω2π(sin(ω)+cos(ωx)01+3sin(2ω)2sin(ω)+cos(ωx)12)=1ω2π(sin(ω)+cos(ω)1+3sin(2ω)2sin(ω)+cos(2ω)cos(ω))=1ω2π(sin(ω)+3sin(2ω)+cos(2ω)1)fs(ω)=2πf(x)sin(ωx)dx=2π01xsin(ωx)dx+2π12(x+1)sin(ωx)dx+2π20×sin(ωx)dx=1ω2π(01xd(cos(ωx))+12(x+1)d(cos(ωx)))=1ω2π(xcos(ωx)01+01cos(ωx)dx(x+1)cos(ωx)12+12cos(ωx)dx)=1ω2π(cos(ω)+sin(ωx)ω013cos(2ω)+2cos(ω)+sin(ωx)ω12)=1ω2π(cos(ω)+sin(ω)ω3cos(2ω)+2cos(ω)+sin(2ω)ωsin(ω)ω)=1ω2π(cos(ω)3cos(2ω)+sin(2ω)ω)\displaystyle (i)\\ \textsf{Fourier sine transform of}\,f(x) \\ \begin{aligned} f_s(\omega) &= \sqrt{\frac{2}{\pi}}\int_{-\infty}^{\infty} f(x) \sin(\omega x) \mathrm{d}x\\ &=\sqrt{\frac{2}{\pi}}\int_0^{10} x^2 \sin(\omega x) \mathrm{d}x + \sqrt{\frac{2}{\pi}}\int_{10}^{\infty} 0 \times \sin(\omega x) \mathrm{d}x\\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\int_0^{10} x^2 \mathrm{d}(-\cos(\omega x))\\ &= \sqrt{\frac{2}{\pi \omega^2}}\left(-x^2 \cos(\omega x)\vert_0^{10} + 2\int_0^{10} x\cos(\omega x) \mathrm{d}x\right)\\ &= \sqrt{\frac{2}{\pi \omega^2}}\left(-100 \cos(100\omega) + \frac{2}{\omega}\int_0^{10} x\mathrm{d}(\sin(\omega x))\right)\\ &= \sqrt{\frac{2}{\pi \omega^2}}\left(-100 \cos(100 \omega) + \frac{2}{\omega} x\sin(\omega x)\vert_0^{10} - \frac{2}{\omega}\int_0^{10} \sin(\omega x) \mathrm{d}x\right)\\ &= \sqrt{\frac{2}{\pi \omega^2}}\left(-100 \cos(100 \omega) + \frac{2}{\omega}x\sin(\omega x)\vert_0^{10} + \frac{2}{\omega^2}\cos(\omega x)\vert_0^{10}\right)\\ &= \sqrt{\frac{2}{\pi \omega^2}}\left(-100 \cos(100 \omega) + \frac{20}{\omega}\sin(20\omega) + \frac{2}{\omega^2}(\cos(10 \omega) - 1)\right) \end{aligned} \\ \textsf{Fourier cosine transform of}\,f(x)\\ \begin{aligned} f_c(\omega) &= \sqrt{\frac{2}{\pi}}\int_{-\infty}^{\infty} f(x) \cos(\omega x) \mathrm{d}x\\ &=\sqrt{\frac{2}{\pi}}\int_0^{10} x^2 \cos(\omega x) \mathrm{d}x + \sqrt{\frac{2}{\pi}}\int_{10}^{\infty} 0 \times \cos(\omega x) \mathrm{d}x\\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\int_0^{10} x^2 \mathrm{d}(\sin(\omega x))\\ &= \sqrt{\frac{2}{\pi \omega^2}}\left(x^2 \sin(\omega x)\vert_0^{10} - 2\int_0^{10} x\sin(\omega x) \mathrm{d}x\right)\\ &= \sqrt{\frac{2}{\pi \omega^2}}\left(100 \sin(100\omega) + \frac{2}{\omega}\int_0^{10} x\mathrm{d}(\cos(\omega x))\right)\\ &= \sqrt{\frac{2}{\pi \omega^2}}\left(100 \sin(100 \omega) +\frac{2}{\omega} x\cos(\omega x)\vert_0^{10} - \frac{2}{\omega}\int_0^{10} \cos(\omega x) \mathrm{d}x\right)\\ &= \sqrt{\frac{2}{\pi \omega^2}}\left(100 \sin(100 \omega) +\frac{2}{\omega}x\cos(\omega x)\vert_0^{10} - \frac{2}{\omega^2}\sin(\omega x)\vert_0^{10}\right)\\ &= \sqrt{\frac{2}{\pi \omega^2}}\left(100 \sin(100 \omega) + \frac{20}{\omega}\cos(20\omega) - \frac{2}{\omega^2}\sin(10 \omega)\right) \end{aligned} \\ (ii)\\ \textsf{Fourier sine transform of}\,f(x) \\ \begin{aligned} f_c(\omega) &= \sqrt{\frac{2}{\pi}}\int_{-\infty}^{\infty} f(x) \cos(\omega x) \mathrm{d}x\\ &=\sqrt{\frac{2}{\pi}}\int_0^{1} x\cos(\omega x) \mathrm{d}x + \sqrt{\frac{2}{\pi}}\int_1^{2} (x + 1)\cos(\omega x) \mathrm{d}x \\&+ \sqrt{\frac{2}{\pi}}\int_{2}^{\infty} 0 \times \cos(\omega x) \mathrm{d}x \\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(\int_0^{1} x\mathrm{d}(\sin(\omega x)) + \int_1^{2} (x + 1)\mathrm{d}(\sin(\omega x))\right)\\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(x\sin(\omega x)\vert_0^{1} - \int_0^{1} \sin(\omega x) \mathrm{d}x + (x + 1)\sin(\omega x)\vert_1^{2}\right.\\&\left. - \int_1^{2} \sin(\omega x) \mathrm{d}x\right)\\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(\sin(\omega) + \frac{\cos(\omega x)}{\omega}\vert_0^{1} +3\sin(2\omega) \right.\\&\left.- 2\sin(\omega) + \frac{\cos(\omega x)}{\omega}\vert_1^{2}\right)\\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(\sin(\omega) + \frac{\cos(\omega)}{\omega} - \frac{1}{\omega} + 3\sin(2\omega) \right.\\&\left.- 2\sin(\omega) + \frac{\cos(2\omega)}{\omega} - \cos(\omega)\right)\\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(-\sin(\omega) + 3\sin(2\omega) + \frac{\cos(2\omega)}{\omega} - \frac{1}{\omega}\right) \end{aligned} \\ \textsf{Fourier cosine transform of}\,f(x) \\ \begin{aligned} f_c(\omega) &= \sqrt{\frac{2}{\pi}}\int_{-\infty}^{\infty} f(x) \cos(\omega x) \mathrm{d}x \\&=\sqrt{\frac{2}{\pi}}\int_0^{1} x\cos(\omega x) \mathrm{d}x + \sqrt{\frac{2}{\pi}}\int_1^{2} (x + 1)\cos(\omega x) \mathrm{d}x \\&+ \sqrt{\frac{2}{\pi}}\int_{2}^{\infty} 0 \times \cos(\omega x) \mathrm{d}x \\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(\int_0^{1} x\mathrm{d}(\sin(\omega x)) + \int_1^{2} (x + 1)\mathrm{d}(\sin(\omega x))\right)\\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(x\sin(\omega x)\vert_0^{1} - \int_0^{1} \sin(\omega x) \mathrm{d}x + \int (x + 1)\sin(\omega x)\vert_1^{2} \right.\\&\left.- \int_1^{2} \sin(\omega x) \mathrm{d}x\right)\\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(\sin(\omega) + \cos(\omega x)\vert_0^{1} +3\sin(2\omega) \right.\\&\left.- 2\sin(\omega) + \cos(\omega x)\vert_1^{2}\right)\\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(\sin(\omega) + \cos(\omega) - 1 + 3\sin(2\omega) \right.\\&\left.- 2\sin(\omega) + \cos(2\omega) - \cos(\omega)\right)\\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(-\sin(\omega) + 3\sin(2\omega) + \cos(2\omega) - 1\right) \end{aligned} \\ \begin{aligned} f_s(\omega) &= \sqrt{\frac{2}{\pi}}\int_{-\infty}^{\infty} f(x) \sin(\omega x) \mathrm{d}x\\ &=\sqrt{\frac{2}{\pi}}\int_0^{1} x\sin(\omega x) \mathrm{d}x + \sqrt{\frac{2}{\pi}}\int_1^{2} (x + 1)\sin(\omega x) \mathrm{d}x \\&+ \sqrt{\frac{2}{\pi}}\int_{2}^{\infty} 0 \times \sin(\omega x) \mathrm{d}x \\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(\int_0^{1} x\mathrm{d}(-\cos(\omega x)) + \int_1^{2} (x + 1)\mathrm{d}(-\cos(\omega x))\right)\\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(-x\cos(\omega x)\vert_0^{1} + \int_0^{1} \cos(\omega x) \mathrm{d}x - (x + 1)\cos(\omega x)\vert_1^{2} \right.\\&\left.+ \int_1^{2} \cos(\omega x) \mathrm{d}x\right)\\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(-\cos(\omega) + \frac{\sin(\omega x)}{\omega}\vert_0^{1} - 3\cos(2\omega) \right.\\&\left.+2\cos(\omega) + \frac{\sin(\omega x)}{\omega}\vert_1^{2}\right)\\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(-\cos(\omega) + \frac{\sin(\omega)}{\omega} - 3\cos(2\omega) + 2\cos(\omega) \right.\\&\left.+ \frac{\sin(2\omega)}{\omega} - \frac{\sin(\omega)}{\omega} \right)\\ &= \frac{1}{\omega}\sqrt{\frac{2}{\pi}}\left(\cos(\omega) - 3\cos(2\omega) + \frac{\sin(2\omega)}{\omega}\right) \end{aligned}



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