Chỉ ghi lại những cái bị lỗi thôi cho rõ nhé

1) a)=> \(\left[\begin{array}{nghiempt}x+5=0\\3x-y=0\end{array}\right.\)<=> \(\left[\begin{array}{nghiempt}x=-5\\y=-15\end{array}\right.\)

b)\(\left[\begin{array}{nghiempt}x-4=0\\y^2-x=0\end{array}\right.\)<=> \(\left[\begin{array}{nghiempt}x=4\\y=\pm2\end{array}\right.\)

c)\(\left[\begin{array}{nghiempt}x=-y\\2x=y\end{array}\right.\)<=> x=y=0

B=2+1/1.2+2+1/2.3+........+2+1/9.10

B=2.9+1/1.2+1/2.3+........+1/9.10

B=18+9/10

mik nhầm dấu \(\in\)là dấu = nha

A\(\in\left\{1;3;5;7;8;10;12\right\}\)

B\(\in\left\{2;4;6;9;11\right\}\)

C\(\in\left\{2\right\}\)