Máy mk cx có ảnh này:

Help me! Please! thanks.

S=1/1.3.5 +1/3.5.7+...................+1/19.21.23 

---> 4S=4/1.3.5 + 4/ 3.5.7 +.........................+4/19.21.23 

= (1/1.3 -1/3.5 ) + ( 1/3.5 -1/3.7) +...................................+(1/... -1/21.23 ) 

= 1/1.3 -1/21.23 

= 1/3 -1/483 =160/483 

----> S = 160/483 : 4 = 160 / 1932 
(em tự rút gọn nhé! )S=1/1.3.5 +1/3.5.7+...................+1/19.21.23 

---> 4S=4/1.3.5 + 4/ 3.5.7 +.........................+4/19.21.23 

= (1/1.3 -1/3.5 ) + ( 1/3.5 -1/3.7) +...................................+(1/... -1/21.23 ) 

= 1/1.3 -1/21.23 

= 1/3 -1/483 =160/483 

----> S = 160/483 : 4 = 160 / 1932 
(em tự rút gọn nhé! )

M=1/1.3.5 +1/3.5.7+.....+1/19.21.23

 4M=4/1.3.5 + 4/ 3.5.7 +...+4/19.21.23

 = (1/1.3 -1/3.5 ) + ( 1/3.5 -1/3.7) +....+(1/19.21 -1/21.23 ) 

= 1/1.3 -1/21.23

 = 1/3 -1/483

=160/483

⇒ S = 160/483 : 4

= 160 / 1932

=40/438

\(a.\)

\(-\left(-315\right)+\left(-115\right)-105+25\)

\(315-115-105+25\)

\(=215\)

\(b.\)

\(888-\left(-333\right)-222+111\)

\(=888+333-222+111\)

\(=1110\)

\(c.\)

\(-97-15+44-35-12+98\)

\(=-17\)

\(A=\frac{1}{15}+\frac{1}{105}+...+\frac{1}{2145}\)

\(=\frac{1}{1.3.5}+\frac{1}{3.5.7}+...+\frac{1}{11.13.15}\)

\(=\frac{1}{4}\left[\frac{4}{1.3.5}+\frac{4}{3.5.7}+...+\frac{4}{11.13.15}\right]\)

\(=\frac{1}{4}.\left[\frac{1}{1.3}-\frac{1}{3.5}+\frac{1}{3.5}-\frac{1}{5.7}+...+\frac{1}{11.13}-\frac{1}{13.15}\right]\)

\(=\frac{1}{4}.\left[\frac{1}{1.3}-\frac{1}{13.15}\right]=\frac{1}{4}.\left[\frac{1}{3}-\frac{1}{195}\right]=\frac{16}{195}\)

\(\frac{3}{x}+\frac{y}{3}=\frac{5}{6}\Rightarrow\frac{3}{x}=\frac{5-2y}{6}\)

\(\Rightarrow x\left(5-2y\right)=18\)

Từ đó tìm được x,y

[ ( 9 - 18 ) + ( -17 ) ] + ( -15 )

= [ ( -9 ) + ( -17 ) ] + ( -15 )

= ( -26 ) + ( -15 )

= -41

666 - ( -442 ) - 100 - 88

= 666 + 442 - 100 - 88

= 1108 - 100 - 88

= 1008 - 88

= 920

- ( -315 ) + ( -115 ) - 105 + 25

= 315 + ( -115 ) - 105 + 25

= 200 - 105 + 25

= 95 + 25

= 120

888 - ( -333 ) - 222 + 70

= 888 + 333 - 222 + 70

= 1221 - 222 + 70

= 999 + 70

= 1069