a

=> $12x$ - $12$ . $5$ + $7.3$ - $7x$ = $15$

=> $12x$ - $60$ + $21$ - $7x$ = $15$

=> $12x$ - $7x$ = $15$ + $60$ - $21$

=> $5x$ = $54$

=>   $x$ = $10,8$

b

=> $30x$ + $30$ . $2$ - $6x$ - $6.5$ - $24x$ = $100$

=> $30x$ + $60$ - $6x$ + $30$ - $24x$ = $100$

=> $30x$ - $6x$ - $24x$ = $100$ - $60$ - $30$

=> $0x$ = $100$

=> $ko$ $có$ $giá$ $trị$ $nào$ $của$ $x$ $để$ $thỏa$ $mãn$ $đề$ $bài$

e)

=> $x$ + $1$ + $3$ + $x$ + $5$ + ...+ $x$ + $99$ = $0$

=> ( $x+x+...+x$ )+ ( $1+3+5+...+99$ ) = $0$

=> $50x$ + $2500$ = $0$

=> $50x$ = $-2500$

=> $x$ = $-50$

g)

=> \(\left[ \begin{array}{l}x=x-3=7\\2y+1=7\end{array} \right.\) 

=> \(\left[ \begin{array}{l}x=7+3\\2y=7-1\end{array} \right.\) 

=>\(\left[ \begin{array}{l}x=10\\2y=6\end{array} \right.\) 

=>\(\left[ \begin{array}{l}x=10\\y=6:2\end{array} \right.\) 

=>\(\left[ \begin{array}{l}x=10\\y=3\end{array} \right.\) 

h) bạn làm tương tự 

Xl nha hơi lâu tại nhóm có vài ng on nên làm hơi lâu mk gửi trc cho bn 5 tấm khì nào xong bảo mk gửi tip tại khum cho quá 5 tấm