\(a=\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+...+\frac{1}{45}\)

\(a=\frac{1}{1.3}+\frac{1}{2.3}+\frac{1}{2.5}+\frac{1}{3.5}+...+\frac{1}{5.9}\)

\(a=2\left(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{9.10}\right)\)

\(a=2\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{9}-\frac{1}{10}\right)\)

\(a=2\left(\frac{1}{2}-\frac{1}{10}\right)\)

=> \(a=2.\frac{2}{5}\)

=> \(a=\frac{4}{5}\)

\(A=\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+\frac{1}{45}\)

\(\Rightarrow\frac{1}{2}A=\left(\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+\frac{1}{45}\right)\cdot\frac{1}{2}\)

\(=\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\)

\(=\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}+\frac{1}{9\cdot10}\)

\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\)

\(=\frac{1}{2}-\frac{1}{10}=\frac{2}{5}\)

\(\Rightarrow A=\frac{2}{5}:\frac{1}{2}=\frac{4}{5}\)

\(a=\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+....+\frac{1}{45}\)

\(a=\frac{1}{1.3}+\frac{1}{2.3}+\frac{1}{2.5}+\frac{1}{3.5}+...+\frac{1}{5.9}\)

\(\frac{1}{2}.a=\frac{1}{2}\left(\frac{1}{1.3}+\frac{1}{2.3}+\frac{1}{2.5}+\frac{1}{3.5}+...+\frac{1}{5.9}\right)\)

\(\frac{1}{2}.a=\frac{1}{2.1.3}+\frac{1}{2.2.3}+\frac{1}{2.2.5}+\frac{1}{2.3.5}+...+\frac{1}{2.5.9}\)

\(\frac{1}{2}.a=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{9.10}\)

\(\frac{1}{2}.a=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{9}-\frac{1}{10}\)

\(\frac{1}{2}.a=\frac{1}{2}-\frac{1}{10}\)

\(\frac{1}{2}.a=\frac{2}{5}\)

\(a=\frac{2}{5}:\frac{1}{2}=\frac{2}{5}.2\)

=> \(a=\frac{4}{5}\)

đố mọi người nha:mik đang làm gì?

2²= 2^x+1

suy ra: 2=x+1

x+1 = 2

x= 2-1=1

1959556529659655654855*1222222/2

7,2 : 2 x 46,5 + 1,55 x 3 x 64

= 3,6 x 46,5 + 4,65 x 64

= 3,6 x ( 4,65 x 10 ) + 4,65 x 64

= 3,6 x 4,65 x 10 + 4,65 x 64

= ( 3,6 x 10 ) x 4,65 + 4,65 x 64

= 36 x 4,65 + 4,65 x 64

= ( 36 + 64 ) x 4,65

= 100 x 4,65

= 465

Nếu mình đúng thì k mình các bạn nhé

7,2 :2 x 46,5 + 1,55 x3 x 64 = 49818.24

=3,6 x 4,65 x 10 + 4.65 x 64 = 4,65 x (36+64)=4,65*100=465

Dễ:

Gọi 3 số tự nhiên liên tiếp cần tìm là:

a;a+1;a+2

Theo bài ra ta có:

a+a+1+a+2=27

a.3+3=27

a.3=27-3

a.3=24

a=24:3

a=8.

Vậy 3 số tự nhiên liên tiếp đó là:

8;9;10.

x = 99 x a -99= 99 x (a-1)= 99 x (1001-1)= 99 x 1000= 99000.

99000

\(6+\frac{4}{12}\)\(-\frac{2}{15}\)

\(C1:6+\left(\frac{1}{3}-\frac{2}{15}\right)\)

\(\frac{90}{15}+\left(\frac{5}{15}-\frac{2}{15}\right)\)\(=\frac{93}{15}\)\(=\frac{31}{5}\)

\(C2:6+\frac{4}{12}-\frac{2}{15}\)

\(6+\frac{1}{3}+\frac{2}{15}\)

\(\frac{90}{15}+\frac{5}{15}-\frac{2}{15}\)

\(b\)Tương tự như câu a em tự giải coi đó như bài tập nha

Phá ngoặc trc nó là dấu cộng thì giữa nguyên dấu

trc nó là dấu trừ thì đổi dấu các số hạng trong ngoặc

\(\)

\(6+\left(\frac{4}{12}-\frac{2}{15}\right)=6+\left(\frac{5}{15}-\frac{2}{15}\right)=\frac{30}{5}+\frac{1}{5}=\frac{31}{5}\)

\(1+\left(\frac{5}{6}+\frac{7}{3}+\frac{2}{12}\right)=1+\left(\frac{10+28+2}{12}\right)=1+\frac{40}{12}=\frac{3}{3}+\frac{10}{3}=\frac{13}{3}\)

y+2xy+3xy+...+10xy=49,5

yx(1+2+3+4+...+10)=49,5

yx45                    =49,5

                          y= 49,5:45

                          y= 1,1

t tôi ik