B ( 13 ) \(\in\){ -1 ; 1 ; -13; 13 }

\(\Rightarrow\)x - 4 = -1 ; 1 ; -13 ; 13

\(\Leftrightarrow\)x = 3 ; 5 ; -9 ; 17

cam on ban nhieu

to lop 5 chiu thoi

mày có bình thường không đấy ?

Bài 2 : 1 + ( -2 ) + 3 + ( -4 )  + ... + 2015

= [ 1 + ( -2 ) ] + [ 3 + ( -4 ) ] + ... + 2015

= -1 + -1 + ... + 2015

Có số các cặp số bằng ( -1 ) là : 

2014 : 2 = 1007 ( cặp ) 

= -1007 + 2015

= 1008

Bài 1 sai đề

\(∘backwin\)

\(a ) ( x + 1 ) + ( x + 2 ) + ( x + 3 ) + ... + ( x + 100 ) = 5750\)

\( ( x + x + x + ... + x ) + ( 1 + 2 + 3 + ... + 100 ) = 5750 \)

\( 100 x + ( 1 + 100 ) ×100 : 2 = 5750\)

\(100 x + 5050 = 5750\)

\( 100 x = 5750 − 5050\)

\(100 x = 700\)

\(x = 700 : 100\)

\(x = 7\)

\(b,\) \(B=\)\(\dfrac{1}{2^2}+\dfrac{1}{3^2}+\dfrac{1}{4^2}+...+\dfrac{1}{2021^2}< \dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+...+\dfrac{1}{2020}+2021\)

\( B < 1 -\)\(\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{2020}-\dfrac{1}{2021}\)

\(B<1-\)\(\dfrac{1}{2021}\)

\(B<\)\(\dfrac{2020}{2021}\)

\(\dfrac{2020}{2021}< 1\)

\(B<1\)

a) (x+1) +(x+2 ) + ...+(x+100)=5750
= 100x + (1+2+3+...+100) = 5750
=100x + 5050 = 5750
--> 100x = 5750-5050=700
--> x=7

từ đó => 8x= 0=> x= 0 t..i..c..k nhé!..!..!..!