x:45:5+34:17=12.43+12.14+12.15+12.17+12.8+12.13

x:45:5+34:17=12.(43+14+15+17+8+13)

x:45:5+34:17=12.110

x:45:5+34:17=1320

x:45:5+2       =1320

x:45:5           =1320-2

x:45:5           =1318

x:45              =1318.5

x:45              =6590

x                   =6590.45

x                   =296550

Ta có : x : 45 : 5 + 34 : 17 = 12.43 + 12.14 + 12.15 + 12.17 + 12.8 + 12.13

                                         = 12. (43 + 14 + 15 +17+ 8 + 13)

                                         = 12.110

=> x : 45 : 5 + 2 = 1320

<=> x : 45 : 5 = 1318

<=> x : 45 = 6950

=> x = 146(6)  

x:45:5+34:17= 12x(43+14+15+17+8+13)

x:45:5+34:17= 12x110

x:45:5+34:17= 1320

tự làm nốt đi, phải động não mới thông minh

7/10 . 11 + 7/11.12 + 7/12.13 + ... + 7/69.70

= 7 ( 1/10.11 + 1/11.12 + 1/12.13 + ... + 1/69 . 70 )

= 7 ( 1/10 - 1/11 + 1/11 - 1/12 + 1/12 - 1/13 + ... + 1/69 - 1/70 )

= 7 ( 1/10 - 1/70 )

= 7 ( 7/70 - 1/70 )

= 7 . 6/70

= 42/70 

= 3/5 

Tk nha !!

=5.167

\(\frac{7}{10.11}+\frac{7}{11.12}+...+\frac{7}{69.70}\)

\(=\frac{7}{10}-\frac{7}{11}+\frac{7}{11}-\frac{7}{12}+...+\frac{7}{69}-\frac{7}{70}\)

\(=\frac{7}{10}-\frac{7}{70}\)

\(=\frac{7}{10}-\frac{1}{10}\)\(=\frac{6}{10}=\frac{3}{5}\)

Đặt \(A=\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)

\(A=\frac{7}{10}-\frac{7}{11}+\frac{7}{11}-\frac{7}{12}+\frac{7}{12}-\frac{7}{13}+...+\frac{7}{69}-\frac{7}{70}\)

\(A=\frac{7}{10}-\frac{7}{70}\)

\(A=\frac{3}{5}\)

\(\frac{7}{10\times11}+\frac{7}{11\times12}+\frac{7}{12\times13}+...+\frac{7}{69\times70}\)

\(=7\times\left(\frac{1}{10\times11}+\frac{1}{11\times12}+\frac{1}{12\times13}+...+\frac{1}{69\times70}\right)\)

\(=7\times\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{69}-\frac{1}{70}\right)\)

\(=7\times\left(\frac{1}{10}-\frac{1}{70}\right)\)

\(=7\times\left(\frac{7}{70}-\frac{1}{70}\right)\)

\(=7\times\frac{6}{70}\)

\(=\frac{3}{5}\)

Chúc bạn học tốt

\(\text{Đặt }A=\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)

\(=7.\left(\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+...+\frac{1}{69.70}\right)\)

\(=7.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+...+\frac{1}{69}-\frac{1}{70}\right)\)

\(=7.\left(\frac{1}{10}-\frac{1}{70}\right)=7.\frac{60}{700}=\frac{3}{5}\)

Vậy A = 3/5.

Barack mới tự học toán 6 hơn 1 tuần, có cách giải này

= 10² + 10 + 11² + 11 + 12² + 12 + 13² + 13 + 14² + 14 + 15² + 15 +16² +16 +17² +17 + 18² + 18 + 19² +19 

= 10² + 11² + 13² + 14² + 15² + 16² + 17² + 18² +19² + 10 + 11 +12 +13 + 14 + 15 + 16 +17 + 18 + 19

Lớp 6 gì xương thế

\(A=7\left(\dfrac{1}{10}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{12}+...+\dfrac{1}{69}-\dfrac{1}{70}\right)\)

\(=7\left(\dfrac{1}{10}-\dfrac{1}{70}\right)=\dfrac{7.60}{700}=\dfrac{420}{700}=\dfrac{3}{5}\)

\(B=\dfrac{1}{2}\left(\dfrac{1}{25}-\dfrac{1}{27}+\dfrac{1}{27}-\dfrac{1}{29}+...+\dfrac{1}{73}-\dfrac{1}{75}\right)\)

\(=\dfrac{1}{2}\left(\dfrac{1}{25}-\dfrac{1}{75}\right)=\dfrac{1}{75}\)

đây

Tham khảo: Câu hỏi của Nguyễn Quốc Đạt - Toán lớp 6 - Học trực tuyến OLM

a)  5 8 .2 = 5 4

b)  4. 6 16 = 3 2

c)  1 2 . 1 3 = 1 6

Lời giải:

$A=12.13+13.14+14.15+...+99.100$

$3A=12.13(14-11)+13.14(15-12)+14.15(16-13)+...+99.100(101-98)$

$=(12.13.14+13.14.15+14.15.16+...+99.100.101) - (11.12.13+12.13.14+13.14.15+...+98.99.100)$
$=99.100.101-11.12.13$

$\Rightarrow A=\frac{99.100.101-11.12.13}{3}$

12.13+14.15+16.17+18.19