@Carl Love 

Thank you very much!

@Rouben Rostamian  Thanks a lot!

@Joe Riel 

Yes, that was what I want. Thank you very much!

@Carl Love 

Thanks for your detailed explanation! It does help me. 

@Carl Love The code is very concise. Thank you so much!

@acer Yes, that is what I wanted. And the suggestion is very nice. Thank you very much! 

@acer Yes, I just want a reformulation with fewer mathematical operation. Thanks for your heip!

@Kevin Dragnet  Thanks a lot!

@tomleslie 

Thank you again!

@mmcdara

Thank you very much! 

@tomleslie 

Thanks for your detailed reply.

1. Actually, I only want to solve the obtained linear equations. Through the results you got, maybe my idea is wrong.

2. Because I am the beginner, so how to put the 51 linear equations together and solve it?(Gennerally, I used solve({eq1,eq2},{v1,v2}) to do it, but the number of linear equtions is too big)