Upward:-4A
Downward:v2-v3/50Ω
Rightward:-1A
Leftward:v2-v1/25Ω
so the second equation should be:-4A +(v2-v3)/50Ω -1A+(v2-v1)/25Ω=0
and the third one:

Upward: (v3-v2)/50Ω
Downward:0
Rightward:-1A
Leftward:v3/20Ω
So the equation should be : (v3-v2)/50Ω-1A+v3/20Ω=0

 

Upward:-4A
Downward:v2-v3/50Ω
Rightward:-1A
Leftward:v2-v1/25Ω

Correct, except for being sloppy with order of operations.

v2-v3/50Ω is v2 - (v3/50Ω) whereas you want (v2-v3)/50Ω.

Get in the habit of being stringent with expressions, otherwise you will end up making easily avoidable mistakes

so the second equation should be:-4A +(v2-v3)/50Ω -1A+(v2-v1)/25Ω=0

Correct.

and the third one:

Upward: (v3-v2)/50Ω
Downward:0
Rightward:-1A
Leftward:v3/20Ω
So the equation should be : (v3-v2)/50Ω-1A+v3/20Ω=0

Very close. The 1A is going away from the Node v3 so it is positive. Your sanity check is that a given current between two nodes is positive for one and negative for the other (because it is going out from one node and into the other).

 

Yes,Thank you very much for this I know solved A ,And All thanks to you ,Do you kn ow how can I begin with b