Bai 2 : 

\(\hept{\begin{cases}x+y=7\\x-7=13\end{cases}\Leftrightarrow\hept{\begin{cases}x+y=7\\x=20\end{cases}}}\)

Thay x vào phương trình đầu ta có : 

\(20+y=7\Leftrightarrow y=-13\)

Vậy \(\left\{x;y\right\}=\left\{20;-13\right\}\)

Thử \(20-13=7\); \(20-7=13\)( thỏa mãn ) 

71 - ( -30) - 37 - 81 + 37 =20

A. 2.9 +6 -4 = 20

B. 8.25 - 6.9 + (-12) = 134

C. -962 + 1548 = 586

D. -85.(36+54) = -85 x 100 = -8500

F. 26.37 - 26.56 - 56.37 + 56.26 =26.37 - 56.37 = 37.(26 - 56)

=37. (-30) = -1100

E. 130 - ( 27 - (36 + 128).2))3

= 130 - ( 27 - 328).3

= 130 - (-301).3

= 130 - (-903)

= 1033

Khóc

bn nhấn vào đúng 0 sẽ ra đáp án

1,tính

23

24

-185

2, 

x+ 1 là số nguyên âm lớn nhất

=> x+1= (-1)

 x = (-1)-1

x= ( -2)

bài 1 dễ tự làm nhé

bài 2: x= -2

hừm

c) (-37) – 14 – 26 + 37=(-37)+37-14-26=0-(-12)=12
d) 34 + 35 + 36 + 37 – 14 – 15 – 16 – 17 =(34-14)+(35-15)+(36-16)+(37-17)=20+20+20+20=80

1.\(\frac{4}{3.7}+\frac{4}{7.11}+\frac{4}{11.15}+\frac{4}{15.19}+\frac{4}{19.23}+\frac{4}{23.27}\)

\(=\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+...+\frac{4}{23}-\frac{4}{27}\)

\(=\frac{1}{3}-\frac{1}{27}=\frac{9}{27}-\frac{1}{27}=\frac{8}{27}\)

2. Đặt \(A=\frac{3}{14}+\frac{3}{84}+\frac{3}{204}+\frac{3}{374}+\frac{3}{594}+\frac{3}{864}\)

\(\Rightarrow A=\frac{3}{2.7}+\frac{3}{7.12}+...+\frac{3}{27.32}\)

\(\Rightarrow5A=3.\left(\frac{5}{2.7}+\frac{5}{7.12}+...+\frac{5}{27.32}\right)\)

\(\Rightarrow5A=3.\left(\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{12}+...+\frac{1}{27}-\frac{1}{32}\right)\)

\(\Rightarrow5A=3.\left(\frac{1}{2}-\frac{1}{32}\right)\)

\(\Rightarrow5A=3.\frac{15}{32}=\frac{45}{32}\Rightarrow A=\frac{45}{32}:5=\frac{9}{32}\)

3. Đặt \(S=\frac{1}{10}+\frac{1}{40}+\frac{1}{88}+...+\frac{1}{340}\)

\(\Rightarrow3S=\frac{3}{10}+\frac{3}{40}+...+\frac{3}{340}\)

\(\Rightarrow3S=\frac{3}{2.5}+\frac{3}{5.8}+...+\frac{3}{17.20}\)

\(\Rightarrow3S=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+...+\frac{1}{17}-\frac{1}{20}\)

\(\Rightarrow3S=\frac{1}{2}-\frac{1}{20}=\frac{9}{20}\Rightarrow S=\frac{9}{20}:3=\frac{3}{20}\)

Câu 1:

\(\frac{4}{3.7}+\frac{4}{7.11}+\frac{4}{11.15}+\frac{4}{15.19}+\frac{4}{19.23}+\frac{4}{23.27}\)

\(\Rightarrow\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{15}+\frac{1}{15}-\frac{1}{19}+\frac{1}{19}-\frac{1}{23}+\frac{1}{23}-\frac{1}{27}\)

\(\Rightarrow\frac{1}{3}-\frac{1}{27}\)

\(=\frac{8}{27}\)

Câu 1:

\(\frac{4}{3\cdot7}+\frac{4}{7\cdot11}+\frac{4}{11\cdot15}+...+\frac{4}{23\cdot27}\)

Áp dụng tính chất \(\frac{b}{a\left[a+b\right]}=\frac{1}{a}-\frac{1}{a+b}\), ta có:

\(\frac{4}{3\cdot7}+\frac{4}{7\cdot11}+\frac{4}{11\cdot15}+...+\frac{4}{23\cdot27}=\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{15}+...+\frac{1}{23}-\frac{1}{27}\)

\(=\frac{1}{3}-\frac{1}{27}=\frac{9}{27}-\frac{1}{27}=\frac{8}{27}\)

Câu 2 tương tự nhưng phải phân

\(\frac{3}{14}+\frac{3}{84}+\frac{3}{204}+...+\frac{3}{864}=\frac{3}{2\cdot7}+\frac{3}{7\cdot12}+\frac{3}{12\cdot17}+...+\frac{3}{27\cdot32}\)

Cái này áp dụng công thức \(\frac{a}{b\left[b+c\right]}=\frac{a}{c}\left[\frac{1}{b}-\frac{1}{b+c}\right]\), ta có:

\(\frac{3}{2\cdot7}+\frac{3}{7\cdot12}+\frac{3}{12\cdot17}+...+\frac{3}{27\cdot32}=\frac{3}{5}\left[\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{12}+\frac{1}{12}-\frac{1}{17}+...+\frac{1}{27}-\frac{1}{32}\right]\)

\(=\frac{3}{5}\left[\frac{1}{2}-\frac{1}{32}\right]=\frac{3}{5}\cdot\frac{15}{32}=\frac{9}{32}\)

Câu 3:

tương tự quy laautj mẫu là 2.5; 5.8 ....

Câu 4: qL mẫu là 1.7; 7.13; ....

Câu 5: \(=\left[1-\frac{1}{2}\right]+\left[1-\frac{1}{6}\right]+...+\left[1-\frac{1}{110}\right]\)

\(=\left[1-\frac{1}{1\cdot2}\right]+\left[1-\frac{1}{2.3}\right]+\left[1-\frac{1}{3\cdot4}\right]+...+\left[1-\frac{1}{10.11}\right]\)

\(=10-\frac{9}{10}=\frac{91}{10}\)

(-37)+14+26+37

=(37-37)+(14+26)

=0+40

=40

(-37) + 14 + 26 + 37

= [(-37) + 37] + 14 + 26

= 0 + 14 + 26

= 14 + 26

= 40

(-37)+14+26+37

=-37+37+(14+26)

=0+40

=40

( - 37 ) + 14 + 26 + 37

= ( 37 - 37 ) + ( 14 + 26 )

= 0 + 40

= 40

=40 nhé!

[ - 37 ] + 14 + 26 + 37 

= { [ - 37 ] + 37 } + { 14 + 26 }

= 0 + 40

= 40

tk cho mk nha