120: 2/3 ?
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)
a, 120 x 3 < 120 x 4
b, 18: (2 x 3)= 18:6=3 = 18:2:3=9:3=3
c, 120:3 > 120:4
d, 14 x 8= 112 = 7 x 16 = 112
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
1) 120 + 20 : 4 - 22.3
=> 120 + 5 - 12
=> 125-12
=>113
2) ( 120 + 20 ) : 4 - 22 . 3
=> 140 : 4 - 12
=> 35 - 12
=> 23
3) 120 + ( 20 : 4 - 22 ).3
=> 120 + 1 . 3
=> 120 + 3
=> 123
1)120+20:4-2\(^2\) .3=120+5-4.3=120+5-12=113
2)(120+20):4-2\(^2\) .3=140:4-4.3=35-12=23
3)120+(20:4-2\(^2\) ).3=120+(5-4).3=120+1.3=120+3=123
![](https://rs.olm.vn/images/avt/0.png?1311)
k cho minh nha bạn
c) =(1+2)+(2^2+2^3)+(2^4+2^5)+...+(2^119+2^200)
=1.(1+2)+2^2.(1+2)+2^4.(1+2)+...+2^119.(1+2)
=1.3+2^2.3+2^4+...+2^199.3 hiển nhiên sẽ chia hết cho 3
Câu d làm tương tự nhưng bạn phải giép 4 lũy thừa để được 15
![](https://rs.olm.vn/images/avt/0.png?1311)
Tổng quát:
\(\dfrac{1}{\left(n+1\right)\sqrt{n}+n\sqrt{n+1}}\)\(=\dfrac{1}{\sqrt{n\left(n+1\right)}\left(\sqrt{n+1}+\sqrt{n}\right)}\)
\(=\dfrac{\sqrt{n+1}-\sqrt{n}}{\sqrt{n\left(n+1\right)}}\)\(=\dfrac{1}{\sqrt{n}}-\dfrac{1}{\sqrt{n+1}}\)
\(\Rightarrow S=\dfrac{10}{11}\)
Ta có công thức tổng quát như sau:
\(\dfrac{1}{\left(n+1\right)\sqrt{n}+n\sqrt{n+1}}\)
\(=\dfrac{\left(n+1\right)\sqrt{n}-n\sqrt{n+1}}{\left[\left(n+1\right)\sqrt{n}+n\sqrt{n+1}\right]\left[\left(n+1\right)\sqrt{n}-n\sqrt{n+1}\right]}\)
\(=\dfrac{\left(n+1\right)\sqrt{n}-n\sqrt{n+1}}{n\left(n+1\right)^2-n^2\left(n+1\right)}\)
\(=\dfrac{\sqrt{n}}{n}-\dfrac{\sqrt{n+1}}{n+1}\)
\(=\dfrac{1}{\sqrt{n}}+\dfrac{1}{\sqrt{n+1}}\)
Áp dụng vào tổng S ta có:
\(S=\dfrac{1}{2\sqrt{1}+1\sqrt{2}}+\dfrac{1}{3\sqrt{2}+2\sqrt{3}}+...+\dfrac{1}{121\sqrt{120}+120\sqrt{121}}\)
\(S=\dfrac{1}{\sqrt{1}}-\dfrac{1}{\sqrt{2}}+\dfrac{1}{\sqrt{2}}-\dfrac{1}{\sqrt{3}}+...+\dfrac{1}{\sqrt{120}}+\dfrac{1}{\sqrt{121}}\)
\(S=1-\dfrac{1}{\sqrt{121}}=1-\dfrac{1}{11}=\dfrac{10}{11}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{120}{1,2x}+\frac{2}{3}=\frac{120}{x}\Rightarrow\frac{100}{x}+\frac{2}{3}=\frac{120}{x}\)
\(\Rightarrow\frac{20}{x}=\frac{2}{3}\Rightarrow x=\frac{20.3}{2}=30\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(B=\left(120+20\right):4-2^2.3\)
\(B=140:4-4.3\)
\(B=35-12\)
\(B=23\)
Tick mik nha
\(A=120+20:4-2^2.3\)
\(A=120+5-4.3\)
\(A=120+5-12\)
\(A=125-12\)
\(A=113\)
180 nhé☺
120: 2/3 =80