@Axel Vogt 

Thanks a lot for your help.

@Axel Vogt 

Thanks.

This way compute \lamda2 correctly but for Vbar(s), It's not helpful.

As @Mariusz Iwaniuk refered to it, Maple can't compute Vbar(s).

@Mariusz Iwaniuk 

Thanks. 

Yes it seems that it's a weakness. 

 Before, I had written this code in Mathematica and had obtained the same result like you. But I tried it in Maple and Maple couldn't solve it. Now, I got ,it's the weakness of Maple.

@Mariusz Iwaniuk 

Thank you Mariusz for your attention.

your solution was very helpful. But I should put A = (t-s) k. when I put it , \lambda2 is computed correctly but there is another problem and it's Vbar(s). It can't be computed correctly .

Do you have any suggestions for computing Vbar(s) symbolically?