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