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1.
a, => 21-x+3 < 0
=> 24-x < 0
=> x < 24
b, => 7+x > 0
=> x > -7
c, => x-1 < 0 ; x+2 > 0 ( vì x-1 < x+2 )
=> x < 1 ; x > -2
=> -2 < x < 1
Tk mk nha
Bài 1:
a) dễ, tự làm :)))
b) \(B=2^{100}-2^{99}-2^{98}-...-2^2-2^1-1.\)
\(\Rightarrow B=2^{100}-\left(2^{99}+2^{98}+2^{97}+...+2^2+2^1+1\right).\)
Đặt: \(M=2^{99}+2^{98}+2^{97}+...+2^2+2^1+1.\)
\(\Rightarrow2M=2\left(2^{99}+2^{98}+2^{97}+...+2^2+2^1+1\right).\)
\(\Rightarrow2M=2^{100}+2^{99}+2^{98}+...+2^3+2^2+2^1.\)
\(\Rightarrow2M-M=\left(2^{100}+2^{99}+2^{98}+...+2^3+2^2+2^1\right)-\left(2^{99}+2^{98}+2^{97}+...+2^2+2^1+1\right).\)
\(\Rightarrow M=2^{100}-1.\)
Ta có: \(B=2^{100}-\left(2^{99}+2^{98}+2^{97}+...+2^2-2^1-2\right).\)
\(\Rightarrow B=2^{100}-\left(2^{100}-1\right).\)
\(\Rightarrow B=\left(2^{100}-2^{100}\right)+1.\)
\(\Rightarrow B=1.\)
Vậy..........
Bài 2:
a) \(\left(x-1\right)\left(x-5\right)< 0.\)
\(\Rightarrow x-1\) và \(x-5\) trái dấu.
mà \(x-1>x-5.\)
\(\Rightarrow\left[{}\begin{matrix}x-1>0.\\x-5< 0.\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x>1.\\x< 5.\end{matrix}\right.\Leftrightarrow1< x< 5.\)
mà \(x\in Z.\)
\(\Rightarrow x\in\left\{2;3;4\right\}.\)
Vậy..........
b) \(\left(x^2-25\right)\left(x^2-5\right)< 0.\)
\(\Rightarrow x^2-25\) và \(x^2-5\) trái dấu.
mà \(x^2-25< x^2-5.\)
\(\Rightarrow\left[{}\begin{matrix}x^2-25< 0.\\x^2-5>0.\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x^2< 25.\\x^2>5.\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x< 5.\\x>\sqrt{5}\left(loại\right).\end{matrix}\right.\Rightarrow x< 5.\)
Vậy..........
a) Vì x thuộc Z và -15 < x < 14
=> x = { -14 ; -13 ; ... ; 12 ; 13 ]
Tổng các số nguyên x : (-14) + (-13) + ... + 12 + 13
= -14 + { (-13) + 13 } + { (-12) + 12 } + ... + 0
= -14
b) 1 + 2 - 3 - 4 + 5 + 6 - 7 - 8 + 9 + ... - 99 - 100 + 101 + 102
= 1 + ( 2 - 3 - 4 + 5 ) + ( 6 - 7 - 8 + 9 ) + ... + ( 98 - 99 - 100 + 101 ) + 102
= 1 + 0 + 0 + ... + 102
= 103
a) Ta có:
(n-1)/n < n/(n+1)
vì (n-1).(n+1)=n2-1 < n2
=>
1/2 < 2/3
3/4 < 4/5
....
99/100 < 100/101
Vậy A < B
b). Ta lại có:
A.B = 1/2 . 2/3 . 3/4 . 4/5 .... . 99/100 . 100/101 = 1/100
Mà A<B => A.A<A.B=1/100
=> A2 < 1/100
=> A < 1/10<1
\(\text{1: 1+(-2)+3+(-4)+........+19+(-20)}\)
\(\text{=1-2+3-4+.....+19-20}\)
\(\text{=-1+(-1)+..........+(-1)}\)
\(\text{=-1.(20-1)+1:2}\)
\(\text{=-1.10}\)
\(=-10\)
\(\text{2: 1- 2 +3 -4+......+99 - 100}\)
\(\text{=( 1- 2) +(3 -4)+......+(99 - 100)}\)
\(\text{=-1+(-1)+.....+(-1)}\)
\(\text{=-1.(100-1)+1:2}\\ \text{=-1.50}\)
\(=-50\)
\(\text{3: 2 - 4+6 - 8+.....+48 - 50}\\ \text{=(2 - 4)+(6 - 8)+.....+(48 - 50)}\\ \text{=-2+(-2)+......+(-2)}\\ \text{=-2.(50-2):2+1}\\ \text{=-2.25}\\ =-50\)
\(\text{4: -1 + 3 - 5 +7 -.....+97 - 99}\\ \text{=-1 +(3-5)+(7-9)+.......+(97-99)}\\ \text{=-1+(-2)+(-2)+........+(-2)}\\ \text{=-1+[(-2).(99-3):2+1]}\\ \text{=-1+[(-2).49]}\\ \text{=-1+(-98)}\\ =-99\)
\(\text{5: 1 + 2 – 3 – 4 + . . . . + 97 + 98 – 99 – 100}\)
\(\text{=1−2+3−4+5−6+...+99−100}\)
\(\text{=(1−2)+(3−4)+(5−6)+...+(99−100) }\)
\(\text{=(−1)+(−1)+(−1)+...+(−1)}\)
\(\text{=(−1).50}\)
\(\text{=−50}\)
1/ 1 + (-2) + 3 + (-4) + . . . + 19 + (-20)
=(1-2)+(3-4)+..........+(19-20)
=-1+(-1)+.......+(-1)
=-1.[(20-1)+1:2]
=-1.10
=-10
2/ 1 – 2 + 3 – 4 + . . . + 99 – 100
=-1+(-1)+....+(-1)
=-1.[(100-1)+1:2]
=-1.50
=-50
3/ 2 – 4 + 6 – 8 + . . . + 48 – 50
=-2+(-2)+.......+(-2)
=-2.[(50-2):2]:2
=-2.12
=-24
=-2.