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tham khảo
https://olm.vn/hoi-dap/tim-kiem?q=A=2100-299-298-297-.........-22-2-1+.+t%C3%ADnh+A&id=52301
\(A=2^{100}-2^{99}-2^{98}-...-2\)
\(\Rightarrow-2A=-2^{101}+2^{100}+2^{99}+...+2^2\)
\(\Rightarrow A-2A=2^{100}-2^{99}-...-2-2^{101}+2^{100}+...2^2\)
\(\Rightarrow-A=2^{100}+2^{100}-2^{101}-2\)
\(\Rightarrow-A=-2\Rightarrow A=2\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=2^{100}-\left(2^{99}+2^{98}+...+2+1\right)\)
Đặt \(B=2^{99}+2^{98}+...+2+1\)
\(\Rightarrow2B=2^{100}+2^{99}+...+2^2+2\)
\(\Rightarrow2B-B=2^{100}-1\Leftrightarrow B=2^{100}-1\)
\(\Rightarrow A=2^{100}-\left(2^{100}-1\right)=1\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=2^{100}-2^{99}+2^{98}-2^{97}+....-2^3+2^2-2+1\\ A=\left(2^{100}+2^{98}+...+2\right)-\left(2^{99}+2^{97}+...+1\right)\)
Gọi \(\left(2^{100}+2^{98}+...+2\right)\)là B
\(B=\left(2^{100}+2^{98}+...+2\right)\\ 2B=2^{102}+2^{100}+.....+2^2\\ 2B-B=\left(2^{102}+2^{100}+.....+2^2\right)-\left(2^{100}+2^{98}+...+2\right)\\ B=2^{102}-2\)
Gọi \(\left(2^{99}+2^{97}+...+1\right)\) là C
\(C=\left(2^{99}+2^{97}+...+1\right)\\ 2C=2^{101}+2^{99}+....+2\\ 2C-C=\left(2^{101}+2^{99}+9^{97}+...+2\right)-\left(2^{99}+9^{97}+...+1\right)\\ C=2^{101}-1\)
\(A=B+C\\ =>A=2^{102}-2+2^{101}-1\\ A=2^{101}\left(2+1\right)-3\\ A=2^{101}\cdot3-3\\ A=3\cdot\left(2^{101}-1\right)\)
\(\dfrac{1}{2}A=2^{99}-2^{98}+...-1+\dfrac{1}{2}\\ \Rightarrow A-\dfrac{1}{2}A=2^{100}-\dfrac{1}{2}\\ \Rightarrow A=2^{101}-1\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(2^{100}-2^{99}+2^{98}-2^{97}+2^{96}-2^{95}+...+2^4-2^3+2^2\)
\(=\left(2^{100}-2^{99}+2^{98}\right)-\left(2^{97}-2^{96}+2^{95}\right)+...+\left(2^4-2^3+2^2\right)\)
\(=2^{96}\left(2^4-2^3+2^2\right)-2^{93}\left(2^4-2^3+2^2\right)+...+\left(2^4-2^3+2^2\right)\)
\(=12\left(2^{96}-2^{93}+...+1\right)⋮12\)
![](https://rs.olm.vn/images/avt/0.png?1311)
C=1-2-3+4+5-6-7+8+...-297-298-299+300+301
=(1-2-3+4)+(5-6-7+8)+...+(297-298-299+300)+301 (75 cặp)
= 0 + 0 +...+ 0 +301 (75 chữ số 0)
= 301
Vậy C=301
![](https://rs.olm.vn/images/avt/0.png?1311)
1+2-3-4+5+6-7-8 +.............+297+298-299-300
= (1+2)-(3+4)+(5+6-(7+8)+.........+(297+298)-(299+300)
=3-7+11-15+.......+595-599
= (3+11+......+595)-(7+15+...........+599)
=22425-22725
= (-300)
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a^2 + b^2 >= ab
<=> a^2 + b^2 -ab >= 0
<=> a^2 - ab + (1/4)b^2 + (3/4)b^2 >= 0
<=> {a - (1/2)b}^2 + (3/4)b^2 >=0
{a - (1/2)b}^2 luôn >= 0
(3/4)b^2 luôn >=0 ==> a^2+b^2 luôn >=0
Bài toán của bạn đưa về giải bất đẳng thức
a^2 + b^2 >= ab
<=> a^2 + b^2 -ab >= 0
<=> a^2 - ab + (1/4)b^2 + (3/4)b^2 >= 0
<=> {a - (1/2)b}^2 + (3/4)b^2 >=0
{a - (1/2)b}^2 luôn >= 0
(3/4)b^2 luôn >=0 ==> a^2+b^2 luôn >=0
* Lưu ý: ab = 2.(1/2).ab
b^2 = (1/4).b^2 + (3/4).b^2
![](https://rs.olm.vn/images/avt/0.png?1311)