2/11.13+2/13.15+...+2/19.21x+23/231=5/77
1/11-1/13+1/13-1/15+...+1/19-1/21x+23/231=5/77
1/11+(-1/13+1/13)+(-1/15+1/15)+...+(-1/19+1/19)-1/21x+23/231=5/77
1/11+0+0+0+...+0-1/21x+23/231=5/77
1/11-1/21x+23/231=5/77
21/231-11/231x+23/231=5/77
10/231x+23/231=5/77
10/231x =5/77-23/231
10/231x =15/231-23/231
10/231x =(-8)/231
x =-8/231:10/231
x =-4/5
vậy x=-4/5
ở trên mình tính lộn.nhớ ****