第 12 类 RD Sharma 解 – 第 22 章微分方程 – 练习 22.3 |设置 1
问题1:证明y=be x +ce 2x是微分方程的解。
d 2 y/dx 2 -3(dy/dx)+2y=0
解决方案:
y=bex+ce2x (i)
Differentiating equation (i)w.r.t x,
dy/dx=bex +2ce2x
dy/dx=bex+2ce2x (ii)
Again, differentiating equation (ii)w.r.t x,
d2y/dx2 =bex+4ce2x (iii)
we have,
d2y/dx2 -3(dy/dx)+2y=0 (iv)
Putting the values ofd2 y/dx2 anddy/dx in equation (iv)
=bex+4ce2x-3(be2x+2ce2x)+2(bex+ce2x)
=3bex-3bex+6ce2x-6ce2x
=0
So,d2y/dx2-3(dy/dx)+2y=0
问题 2:验证 y=4sin3x 是微分方程的解。
d 2 y/dx 2 +9y=0
解决方案:
y=4sin3x (i)
Differentiating equation (i)w.r.t x,
dy/dx=(4)(3)cos3x (ii)
Again differentiating equation (ii)w.r.t x,
d2y/dx2 =-(12)(3)sin3x
d2y/dx2=-(9)(4sin3x)
d2y/dx2=-9y (Since y=4sin3x)
d2y/dx2+9y=0
So, d2y/dx2+9y=0
问题 3:证明 y=ae 2x +be -x 是微分方程的解。
d 2 y/dx 2 -dy/dx-2y=0
解决方案:
y=ae2x+be−x (i)
Differentiating equation (i)w.r.t x,
dy/dx=2ae2x-be-x (ii)
Again differentiating equation (ii)w.r.t x,
d2y/dx2=4ae2x+be-x (iii)
we have,
d2y/dx2-dy/dx-2y (iv)
Putting the values ofandin equation (iv)
=4ae2x+be-x-(2ae2x-be-x)-2(ae2x+be−x)
=4ae2x-4ae2x +be−x-be−x)
=0
问题 4:证明函数y=Acosx-Bsinx 是微分方程的解。
d 2 y/dx 2 +y=0
解决方案:
y=Acosx-Bsinx (i)
Differentiating equation (i)w.r.t x,
dy/dx=-Asinx-Bcosx (ii)
Again differentiating equation (ii)w.r.t x,
d2y/dx2=-Acosx+Bsinx
d2y/dx2=-(Acosx-Bsinx)
d2y/dx2+(Acosx-Bsinx)=0
d2y/dx2+y=0 (since y=Acosx-Bsinx)
问题 5:证明函数y=Acos2x-Bsin2x 是微分方程的解。
d 2 y/dx 2 + 4y = 0
解决方案:
y=Acos2x-Bsin2x (i)
Differentiating equation (i)w.r.t x,
dy/dx=-2Asin2x-2Bcos2x (ii)
Again differentiating equation (ii)w.r.t x,
d2y/dx2=-4Acos2x+4Bsin2x
d2y/dx2+4(Acos2x-Bsin2x)=0
d2y/dx2+4y=0 (since y=Acos2x-Bsin2x)
问题6:证明,y=Ae Bx是微分方程的解。
d 2 y/dx 2 =(1/y)(dy/dx) 2
解决方案:
y=AeBx (i)
Differentiating equation (i)w.r.t x,
dy/dx=ABeBx (ii)
Again differentiating equation (ii)w.r.t x,
d2y/dx2=AB2ebx
d2y/dx2=(ABebx)2 /(AeBx)
d2y/dx2=(1/y)(dy/dx)2
问题 7:验证 y= (x/a)+b 是微分方程的解。
d 2 y/dx 2 +(2/x)(dy/dx) 2 =0
解决方案:
y= (x/a)+b (i)
Differentiating equation (i)w.r.t x,
dy/dx=-(a/x2) (ii)
Again differentiating equation (ii)w.r.t x,
d2y/dx2=+(2a/x3)
d2y/dx2=-(-2/x)(a/x2)
d2y/dx2+(2/x)(dy/dx)=0
问题8:验证y 2 =4ax 是微分方程的解。
x(dy/dx)+y(dx/dy)=y
解决方案:
y2=4ax (i)
Differentiating equation (i)w.r.t x,
2y(dy/dx)=4a
dy/dx=(2a/y)
we have,
x(dy/dx)+y(dx/dy)
=x(2a/y)+y(y/2a)
=(4xa+y2)/2y
=(2y2/2y)
=y
问题 9:证明 Ax 2 +By 2 =1 是微分方程的解。
解决方案:
Ax2+By2=1 (i)
Differentiating equation (i)w.r.t x,
2Ax+2By(dy/dx)=0
2Ax=-2By(dy/dx)
y(dy/dx)=-(Ax/B) (ii)
Again differentiating equation (ii)w.r.t x,
(dy/dx)2+y(d2y/dx2)=-(A/B)
(dy/dx)2+y(d2y/dx2)=-(y/x)(dy/dx)
问题 10:证明 y=ax 3 +bx 2 +cis 是微分方程的解。
(d 3 y/dx 3 )=6a
解决方案:
We have,
y=ax3+bx2+c (i)
Differentiating equation (i)w.r.t x,
(dy/dx)=3ax2+2bx (ii)
Again differentiating equation (ii)w.r.t x,
(d2y/dx2)=6ax (iii)
Again differentiating equation (iii)w.r.t x,
(d3y/dx3)=6a