Bài 6:
$a,A=1-2+3-4+5-6+...+2017-2018+2019$
$A=(1-2)+(3-4)+(5-6)+...+(2017-2018)+2019$
$A=(-1)+(-1)+(-1)+...+(-1)+2019$
$A=(-1).1009+2019$
$A=-1009+2019$
$A=1010$
$b,B=-1+2-3+4-5+6-...-2017+2018-2019$
$B=-1+(2-3)+(4-5)+(6-7)+....+(2016-2017)+(2018-2019)$
$B=-1+(-1)+(-1)+...+(-1)+(-1)$
$B=(-1).2019$
$B=-2019$
Bài 7:
$a)(x-15)(x-17)=0$
⇔\(\left[ \begin{array}{l}x-15=0\\x-17=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=15\\x=17\end{array} \right.\)
Vây $x \in${$15;17$}
$b)(29-x).5=45$
$⇔29-x=9$
$⇔x=20$
$c)34-3(x-9)=25$
$⇔3(x-9)=9$
$⇔x-9=3$
$⇔x=12$
$d)484+x=-363-(-548)$
$⇔484+x=-363+548$
$⇔484+x=185$
$⇔x=-299$
~ Học tốt ~
