國立中山大學113學年度碩士班招生考試
科目名稱:工程數學【機電系碩士班乙組、丙組】
解答:(a)y=xm⇒y′=mxm−1⇒y″=m(m−1)xm−2⇒x2y″−2xy′+4y=(m2−3m+4)xm=0⇒m2−3m+4=0⇒m=3±√7i2⇒yh=c1x3/2cos(√72lnx)+c2x3/2sin(√72lnx)..待解(b)y=x顯然為其一解,因此令{y1=xy=v+y1=v+x代回原式⇒x(v′+1)−(v+x)+2(v+x)2=2x2⇒xv′−v+2v2+4xv=0⇒v′+(4−1x)v=−2xv2取u=1v⇒u′=−v′v2=−u2v′⇒v′=−u′u2⇒−u′u2+(4−1x)1u=−2x⋅1u2⇒u′+(1x−4)u=2x⇒ integrating factor I(x)=e∫((1/x)−4)dx=xe−4x⇒xe−4xu′+(1−4x)e−4xu=2e−4x⇒(xe−4xu)′=2e−4x⇒xe−4xu=∫2e−4xdx=−12e−4x+c1⇒u=−12x+c1xe4x=c2e4x−12x⇒v=1u=2xc2e4x−1⇒y=v+x⇒y=2xc2e4x−1+x

解答: (a)
解答:u(x,t)=X(x)T(t) \Rightarrow XT''=X''T \Rightarrow {T''\over T}={X''\over X}=\lambda\\ \lambda\ge 0 \Rightarrow X=0 \Rightarrow u=0\\ \lambda\lt 0 \Rightarrow \lambda=-\rho^2 \Rightarrow X''+\rho^2=0 \Rightarrow X=c_1\cos \rho x+c_2 \sin \rho x \Rightarrow \cases{X(0)=c_1=0\\ X(L=\pi)= c_2\sin \rho \pi=0} \\ \Rightarrow \rho =1,2,\cdots \Rightarrow X_n=\sin n x\\ T''+\rho^2 T=0 \Rightarrow T=c_1\cos \rho t+c_2\sin \rho t \Rightarrow T_n=c_1\cos nt+c_2\sin nt \\ \Rightarrow u(x,t)=\sum_{n=1}^\infty (A_n \cos nt+B_n \sin nt)\sin(nx) \\ \Rightarrow u_t(x,t)=\sum_{n=1}^\infty (-nA_n \sin nt+ nB_n \cos nt)\sin(nx) \\ \Rightarrow u_t(x,0) =\sum_{n=1}^\infty nB_n\sin(nx)\\ \Rightarrow B_n={2\over n\pi} \int_0^\pi g(x) \sin(nx)\,dx ={2\over n\pi} \left( \int_0^{\pi/2} 0.01x\sin(nx)\,dx + \int_{\pi/2}^\pi 0.01(\pi-x)\sin(nx)\,dx\right) \\={0.02\over n\pi} \left(\left. \left[{\sin(nx)-nx\cos(nx)\over n^2} \right] \right|_0^{\pi/2} + \left. \left[ -{\sin(nx)+n(\pi-x)\cos(nx) \over n^2}\right] \right|_{\pi/2}^\pi \right) \\={0.02\over n\pi }\cdot {2\sin(n\pi/2) \over n^2} ={0.04\over n^3\pi} \sin(n\pi/2) \Rightarrow B_n={0.04\over n^3\pi} \sin(n\pi/2)\\ u(x,0)=0 \Rightarrow u(x,t)=\sum_{n=1}^\infty A_n \sin(nx)=0 \Rightarrow A_n=0 \\ \Rightarrow \bbox[red, 2pt]{u(x,t)=\sum_{n=1}^\infty {0.04\over n^3\pi} \sin(n\pi/2) \sin (nt) \sin(nx)}
解答:\textbf{(a)}\; \begin{vmatrix}\vec i & \vec j& \vec k\\ \frac{\partial }{\partial x}& \frac{\partial }{\partial y} & \frac{\partial }{\partial z}\\y^2\cos x+z^3 & 2y\sin x-4& 3xz^2+2 \end{vmatrix} =2y\cos x\vec k+3z^2 \vec j-2y\cos x\vec k-3z^2 \vec j=0 \\\qquad \Rightarrow \vec F\text{ is a conservative field}. \bbox[red, 2pt]{QED}\\ \textbf{(b)}\; \text{Let } \nabla\Phi= \mathbf F, \text{ then }\Phi_x= y^2\cos x+z^3 \Rightarrow \Phi =\int (y^2\cos x+z^3)\,dx \\ \Rightarrow \Phi=y^2\sin x+xz^3+ \phi(y,z) \Rightarrow \Phi_y=2y\sin x+\phi_y=2y\sin x-4\\ \Rightarrow \phi_y=-4 \Rightarrow \phi(y,z)=-4y +\rho(z) \Rightarrow \Phi = y^2\sin x+xz^3-4y+\rho(z) \\ \Rightarrow \Phi_z=3xz^2+\rho'(z)=3xz^2+2 \Rightarrow \rho'(z)=2 \Rightarrow \rho(z)=2z+c_1\\ \Rightarrow \bbox[red, 2pt]{\Phi = y^2\sin x+xz^3-4y+2z+ c_1}\\ \textbf{(c)}\; \Phi(\pi/2,-1,2)-\Phi(1,1,-1) =(1+4\pi +4+4) -(\sin 1-1-4-2)=\bbox[red, 2pt]{4\pi +2-\sin 1}

(b)f(t)=π3(u(t)−u(t−2π))+u(t−4π)2πsin(4t)(c)L{f(t)}=∫∞0f(t)e−stdt=∫2π0π3e−stdt+∫∞4π2πsin(4t)e−stdt=π3[−1se−st]|2π0+2π[−e−sts2+16(ssin(4t)+4cos(4t)]|∞0=π3s(1−e−2πs)+2π(4s2+16)
解答:(a)A=[10−2−121−31−24120202]R2−2R1→R2,R3+2R1→R3→[10−2−1011304−300202]R3−4R2→R3,R4−2R2→R4→[10−2−1011300−7−1200−2−4]R3/(−7)→R3→[10−2−1011300112700−2−4]R1+2R3→R1,R2−R3→R2,R4+2R3→R4→[10017701097001127000−47]−(7/4)R4→R4→[100177010970011270001]R1−(17/7)R4→R1,RO2−(9/7)R4→R2,R3−(12/7)R4→R3→[1000010000100001]⇒rref(A)=[1000010000100001]⇒rank(A)=4(b)A=[10−2−121−31−24120202]R2−2R1→R2,R3+2R1→R3→[10−2−1011304−300202]R3−4R2→R3,R4−2R2→R4→[10−2−1011300−7−1200−2−4]R4−(2/7)R3→R4→[10−2−1011300−7−12000−47]⇒det
解答:\textbf{(a)}\; \begin{vmatrix}\vec i & \vec j& \vec k\\ \frac{\partial }{\partial x}& \frac{\partial }{\partial y} & \frac{\partial }{\partial z}\\y^2\cos x+z^3 & 2y\sin x-4& 3xz^2+2 \end{vmatrix} =2y\cos x\vec k+3z^2 \vec j-2y\cos x\vec k-3z^2 \vec j=0 \\\qquad \Rightarrow \vec F\text{ is a conservative field}. \bbox[red, 2pt]{QED}\\ \textbf{(b)}\; \text{Let } \nabla\Phi= \mathbf F, \text{ then }\Phi_x= y^2\cos x+z^3 \Rightarrow \Phi =\int (y^2\cos x+z^3)\,dx \\ \Rightarrow \Phi=y^2\sin x+xz^3+ \phi(y,z) \Rightarrow \Phi_y=2y\sin x+\phi_y=2y\sin x-4\\ \Rightarrow \phi_y=-4 \Rightarrow \phi(y,z)=-4y +\rho(z) \Rightarrow \Phi = y^2\sin x+xz^3-4y+\rho(z) \\ \Rightarrow \Phi_z=3xz^2+\rho'(z)=3xz^2+2 \Rightarrow \rho'(z)=2 \Rightarrow \rho(z)=2z+c_1\\ \Rightarrow \bbox[red, 2pt]{\Phi = y^2\sin x+xz^3-4y+2z+ c_1}\\ \textbf{(c)}\; \Phi(\pi/2,-1,2)-\Phi(1,1,-1) =(1+4\pi +4+4) -(\sin 1-1-4-2)=\bbox[red, 2pt]{4\pi +2-\sin 1}

解答:\vec r(u,v)=[u,v,6u-9v] \Rightarrow \cases{\vec r_u=[1,0,6] \\ \vec r_v=[0,1,-9]} \Rightarrow \vec r_u\times \vec r_v =[-6,9,1]\\ \vec F(u,v)=[ 5u,v^3,0] \Rightarrow \int \vec F\cdot (\vec r_u\times \vec r_v )dudv = \int_{-1}^1 \int_0^2 -30u+9v^3\,dudv \\=\int_{-1}^1 -60+18v^3\,dv= \bbox[red, 2pt]{-120}
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解題僅供參考,其他歷年試題及詳解
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