(Câu b hình như viết dư x hay số 1240 ko chính xác nên ra số xấu rồi)
`b) x+( x+1)+(x+2)+...+(x+80)=1240`
`=>81x+1+2+...+80=1240`
`[1+2+...+80=(1+80)+(2+79)+...+(40+41)=81. 80/2]`
`=>81x+81. 80/2=1240`
`=>81x+3240=1240`
`=>81x=1240-3240`
`=>81x=-2000`
`=>x={-2000}/81`
Vậy `x={-2000}/81`
`c)` `3^x+3^{x+1}+3^{x+2}=3^4 .13`
`=>3^x. (1+3+3^2)=3^4 . 13`
`=>3^x. 13=3^4 .13`
`=>3^x =3^4`
`=> x=4`
Vậy `x=4`
`d)` `|x-3|+2=5`
`=>|x-3|=5-2`
`=>|x-3|=3`
$⇒\left[\begin{array}{l}x-3=3\\x-3=-3\end{array}\right.$
$⇒\left[\begin{array}{l}x=3+3=6\\x=-3+3=0\end{array}\right.$
Vậy `x=0; x=6`
