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2024年4月23日 星期二

113年中山機電碩士班-工程數學詳解

 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=xmy=mxm1y=m(m1)xm2x2y2xy+4y=(m23m+4)xm=0m23m+4=0m=3±7i2yh=c1x3/2cos(72lnx)+c2x3/2sin(72lnx)..(b)y=x,{y1=xy=v+y1=v+xx(v+1)(v+x)+2(v+x)2=2x2xvv+2v2+4xv=0v+(41x)v=2xv2u=1vu=vv2=u2vv=uu2uu2+(41x)1u=2x1u2u+(1x4)u=2x integrating factor I(x)=e((1/x)4)dx=xe4xxe4xu+(14x)e4xu=2e4x(xe4xu)=2e4xxe4xu=2e4xdx=12e4x+c1u=12x+c1xe4x=c2e4x12xv=1u=2xc2e4x1y=v+xy=2xc2e4x1+x
解答: (a)

(b)f(t)=π3(u(t)u(t2π))+u(t4π)2πsin(4t)(c)L{f(t)}=0f(t)estdt=2π0π3estdt+4π2πsin(4t)estdt=π3[1sest]|2π0+2π[ests2+16(ssin(4t)+4cos(4t)]|0=π3s(1e2πs)+2π(4s2+16)

解答:(a)A=[1021213124120202]R22R1R2,R3+2R1R3[1021011304300202]R34R2R3,R42R2R4[10210113007120024]R3/(7)R3[102101130011270024]R1+2R3R1,R2R3R2,R4+2R3R4[1001770109700112700047](7/4)R4R4[100177010970011270001]R1(17/7)R4R1,RO2(9/7)R4R2,R3(12/7)R4R3[1000010000100001]rref(A)=[1000010000100001]rank(A)=4(b)A=[1021213124120202]R22R1R2,R3+2R1R3[1021011304300202]R34R2R3,R42R2R4[10210113007120024]R4(2/7)R3R4[102101130071200047]det


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}


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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