1,
a)-20$-20\leq x$ $\leq20$
⇒ x ∈{-20;-19;.....;19;20}
Ta có:
(-20)+(-19)+....+19+20
=[(-20)+20]+[(-19)+19]+......+[(-1)+1]+0
=0+0+....+0+0
=0
b)0>x>30
⇒x ∈ {1;2;3;.......;18;19}
Ta co:
1+2+3+.....+19( có 19 số hạng)
=[(19+1).19]:2
=[20.19]:2
=380 : 2
=190
2,
|x-2|+1=5
|x-2| =5-1
|x-2| =4
⇒\(\left[ \begin{array}{l}x-2=4\\x-2=-4\end{array} \right.\)
⇒\(\left[ \begin{array}{l}x=4+2\\x=-4+2\end{array} \right.\)
⇒\(\left[ \begin{array}{l}x=6\\x=-2\end{array} \right.\)
Vậy x ∈{-2;6}
Chúc bn học tốt!x
