# Question:Can't solve a differential equation

## Question:Can't solve a differential equation

Maple 2019

I have an ordinary differential equation to be solved. when I give numeric values to B,H and L__1 (for example B=10,H=10,L__1=4), everything works fine and equations eq37a will be solved.

but when B,H and L__1 are symbolic parameters, dsolve give me this error " Error, (in dsolve) give the main variable as a second argument". and I don't know what it means.

is there any way to solve the ODE with symbolic B,H and L__1??

 > restart; #B:=10; #H:=10; #L__1:=4; eq0:=R__i=(1/2)*B+L__1; eq01:=R__f=C+(1/2)*B+L__1; eq02:=theta__1=arctan((H/2)/(B/2 + L__1)); eq03:=theta__2=Pi - arctan((H/2)/(B/2 - L__1)); eq43:= v__beta(r,theta,beta)= v__m(beta)*(1-(r^2/((R__max(theta,beta))^2))); eq39:=R(beta)=R__i+2*beta*(R__f-R__i)/Pi; eq39a:=simplify(subs([eq0,eq01], eq39)); eq38:=R__max(theta,beta)=R__max(theta,0)*(R(beta)/R__i); eq41:=R__max(theta,0)=piecewise(theta<=rhs(eq02),(B/2+L__1)/cos(theta),theta<=rhs(eq03),(H/2)/sin(theta),rhs(eq03)= 2 , H > 0 , B > 0 , L__1 > 0; eq44b:= v__m(beta)= rhs(eq44a) * B * H; eq47:=subs([eq44b,eq38a],eq43);
 (1)
 > eq37:=diff(v__r(r, theta, beta),r)*r*(R__f-r*cos(theta))+v__r(r,theta,beta)*(R__f-2*r*cos(theta))+r*diff(v__beta(r,theta,beta),beta)=0; eq37a:=simplify(expand(subs(eq47, eq37)));
 (2)
 > bc:=v__r(rhs(eq38a),theta,beta)=0;
 (3)
 > sol3:=simplify(dsolve([eq37a,bc], v__r(r, theta,beta)));
 >