1-1+9x9
Hãy nhập câu hỏi của bạn vào đây, nếu là tài khoản VIP, bạn sẽ được ưu tiên trả lời.
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=\dfrac{1}{15}\times\dfrac{15}{13}\times\dfrac{13}{9}\times9\)
\(A=\dfrac{1}{15}\times\dfrac{15}{13}\times\dfrac{13}{9}\times\dfrac{9}{1}\)
\(A=\dfrac{1}{1}\)
\(A=1\)
(Sử dụng phương pháp triệt tiêu(rút gọn)
![](https://rs.olm.vn/images/avt/0.png?1311)
1 + 1 + 1 x 0 = 2
9 x 9 : 9 x 0 + 10 - 2 x 0 = 81 : 9 x 0 + 10 - 2 x 0 = 9 x 0 + 10 - 2 x 0 = 0 + 10 - 2 x 0 = 0 + 10 - 0 = 10
ủng hộ mk nhé mk tk cho cậu!!
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có : A = \(\frac{1}{2.2}+\frac{1}{3.3}+\frac{1}{4.4}+...+\frac{1}{9.9}< \frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{8.9}\)
= \(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{8}-\frac{1}{9}=1-\frac{1}{9}=\frac{8}{9}\)
\(\Rightarrow A< \frac{8}{9}\)(1)
Lại có : \(A=\frac{1}{2.2}+\frac{1}{3.3}+\frac{1}{4.4}+...+\frac{1}{9.9}>\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}\)
\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{9}-\frac{1}{10}=\frac{1}{2}-\frac{1}{10}=\frac{2}{5}\)
\(\Rightarrow A>\frac{2}{5}\)(2)
Từ (1);(2) => \(\frac{8}{9}>A>\frac{2}{5}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
1 + 2 x 2 + 3 x 3 + 4 x 4 + 5 x 5 + 6 x 6 + 7 x 7 + 8 x 8 + 9 x 9 + 10 x 10
= 1 + 4 + 9 + 16 + 25 + 36 + 49 + 64 + 81 + 100
= 385
1 + 2x2 + 3x3 + 4x4 + 5x5 + 6x6 + 7x7 + 8x8 + 9x9 + 10x10
= 1+4+9+16+25+36+49+64+81+100
=(81+9)+(64+16)+(49+1)+)36+4)+25+100
=90+80+50+40+25 +100
=385
![](https://rs.olm.vn/images/avt/0.png?1311)
9 x 9 - 9 x 1 = 9 ( 9 - 1 ) = 9 x 8 = 72
1 + 1 + 1 x 0 = 2 + 0 = 2
9 x 9 x 0 + 10 + 2 x 0 = 0 + 10 + 0 = 10
![](https://rs.olm.vn/images/avt/0.png?1311)
1 - 1 + 9 x 9 = 81 nha
1-1+9x9
=0+81
=81
bằng 81 nha bạn