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\(A=3^{100}-3^{99}+3^{98}-...-3+1\\ \Rightarrow\dfrac{1}{3}A=3^{99}-3^{98}+3^{97}-...-1+\dfrac{1}{3}\\ \Rightarrow\dfrac{4}{3}A=3^{100}+\dfrac{1}{3}\\ \Rightarrow A=\dfrac{3^{101}}{4}+\dfrac{1}{4}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a: \(A=3^{100}-3^{99}+3^{98}-...+3^2-3\)
=>\(3A=3^{101}-3^{100}+3^{99}-...+3^3-3^2\)
=>\(4A=3^{101}-3\)
=>\(A=\dfrac{3^{101}-3}{4}\)
b: \(B=\left(-2\right)^0+\left(-2\right)^1+...+\left(-2\right)^{2024}\)
=>\(B\cdot\left(-2\right)=\left(-2\right)^1+\left(-2\right)^2+...+\left(-2\right)^{2025}\)
=>\(-2B-B=\left(-2\right)^1+\left(-2\right)^2+...+\left(-2\right)^{2025}-\left(-2\right)^0-\left(-2\right)^1-...-\left(-2\right)^{2024}\)
=>\(-3B=-2^{2025}-1\)
=>\(B=\dfrac{2^{2025}+1}{3}\)
c: \(C=\left(-\dfrac{1}{5}\right)^0+\left(-\dfrac{1}{5}\right)^1+...+\left(-\dfrac{1}{5}\right)^{2023}\)
=>\(\left(-\dfrac{1}{5}\right)\cdot C=\left(-\dfrac{1}{5}\right)^1+\left(-\dfrac{1}{5}\right)^2+...+\left(-\dfrac{1}{5}\right)^{2024}\)
=>\(\left(-\dfrac{6}{5}\right)\cdot C=\left(-\dfrac{1}{5}\right)^{2024}-\left(-\dfrac{1}{5}\right)^0\)
=>\(C\cdot\dfrac{-6}{5}=\dfrac{1}{5^{2024}}-1=\dfrac{1-5^{2024}}{5^{2024}}\)
=>\(C\cdot\dfrac{6}{5}=\dfrac{5^{2024}-1}{5^{2024}}\)
=>\(C=\dfrac{5^{2024}-1}{5^{2024}}:\dfrac{6}{5}=\dfrac{5^{2024}-1}{6\cdot5^{2023}}\)
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\(A=\dfrac{1}{3}+\dfrac{1}{3^2}+\dfrac{1}{3^3}+\dfrac{1}{3^4}+...+\dfrac{1}{3^{99}}\)
\(\Rightarrow\dfrac{A}{3}=\dfrac{1}{3^2}+\dfrac{1}{3^3}+\dfrac{1}{3^4}+...+\dfrac{1}{3^{100}}\)
\(\Rightarrow A-\dfrac{A}{3}=\dfrac{2A}{3}=\left(\dfrac{1}{3}+\dfrac{1}{3^2}+\dfrac{1}{3^3}+...+\dfrac{1}{3^{99}}\right)-\left(\dfrac{1}{3^2}+\dfrac{1}{3^3}+\dfrac{1}{3^4}+...+\dfrac{1}{3^{100}}\right)\)
\(\Rightarrow\dfrac{2A}{3}=\left(\dfrac{1}{3^2}-\dfrac{1}{3^2}\right)+\left(\dfrac{1}{3^3}-\dfrac{1}{3^3}\right)+...+\left(\dfrac{1}{3^{99}}-\dfrac{1}{3^{99}}\right)+\left(\dfrac{1}{3}-\dfrac{1}{3^{100}}\right)=\dfrac{1}{3}-\dfrac{1}{3^{100}}\)
\(\Rightarrow2A=3\cdot\left(\dfrac{1}{3}-\dfrac{1}{3^{100}}\right)\)
\(\Rightarrow\text{A}=\dfrac{1-\dfrac{1}{3^{99}}}{2}\)
\(\Rightarrow A=\dfrac{1}{2}-\dfrac{1}{2.3^{99}}< \dfrac{1}{2}\)
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SSH: (399-1):2+1= 200
Neu chia moi nhom 4 so thi so cap so la:
200:4 = 50
Ta co:
B=1+3-5-7+9+11-13-15+...+393+395-397-399
B= (1+3-5-7)+(9+11-13-15)+...+(393+395-397-399)
B= -8 + -8 +...+ -8
B= -8 . 50
B= -400
SSH: (399-1):2+1= 200
Neu chia moi nhom 4 so thi so cap so la:
200:4 = 50
Ta co:
A=1+3-5-7+9+11-13-15+...+393+395-397-399
A= (1+3-5-7)+(9+11-13-15)+...+(393+395-397-399)
A= -8 + -8 +...+ -8
A= -8 . 50
A= -400
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\(A=1+3^2+3^4+...+3^{102}\)
\(9A=3^2+3^4+...+3^{102}+3^{104}\)
\(\Rightarrow9A-A=3^{104}-1\)
\(\Rightarrow8A=3^{104}-1\)
\(\Rightarrow A=\dfrac{3^{104}-1}{8}\)
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Z=31+32+33+34+...+3100
3Z=3.(31+32+33+34+...+3100)
3Z=3.31+3.32+3.33+...+3.3100
3Z=32+33+34+...+3101
Lấy 3Z= 32+33+34+...+3101
-
Z=31+32+33+34+...+3100
------------------------------------------- 2Z=3^101-3 =>Z=(3^101-3):2 Chú thích: ^ là mũ, cái phần đặt tính thì bạn để các số bằng nhau thẳng hàng nhé
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\(A=1-2+3-4+5-6+7-8+...+99-100\)
\(A=\left(-1\right)+\left(-1\right)+\left(-1\right)+\left(-1\right)+...+\left(-1\right)\)
\(A=\left(-1\right).50\)
\(A=-50\)
\(B=1+3-5-7+9+11-...-397-399\)
\(B=1-2+2-2+2-...+2-2-399\)
\(B=1-399\)
\(B=-398\)
\(C=1-2-3+4+5-6-7+...+97-98-99+100\)
\(C=-1+1-1+1-...-1+1\)
\(C=0\)
\(D=2^{2024}-2^{2023}-...-1\)
\(D=2^{2024}-\left(2^0+2^1+2^2+...2^{2023}\right)\)
\(D=2^{2024}-\left(\dfrac{2^{2024}-1}{2-1}\right)\)
\(D=2^{2024}-\left(2^{2024}-1\right)\)
\(D=2^{2024}-2^{2024}+1\)
\(D=1\)
A = 1 - 2 + 3 - 4 + 5 - 6 + 7 - 8 +...+ 99 - 100
A = (1 - 2) + ( 3 - 4) + ( 5- 6) +....+(99 - 100)
Xét dãy số 1; 3; 5;...;99
Dãy số trên là dãy số cách đều có khoảng cách là: 3 - 1 = 2
Dãy số trên có số số hạng là: (99 - 1) : 2 + 1 = 50 (số)
Vậy tổng A có 50 nhóm, mỗi nhóm có giá trị là: 1- 2 = -1
A = - 1\(\times\)50 = -50
b,
B = 1 + 3 - 5 - 7 + 9 + 11-...- 397 - 399
B = ( 1 + 3 - 5 - 7) + ( 9 + 11 - 13 - 15) + ...+( 393 + 395 - 397 - 399)
B = -8 + (-8) +...+ (-8)
Xét dãy số 1; 9; ...;393
Dãy số trên là dãy số cách đều có khoảng cách là: 9-1 = 8
Dãy số trên có số số hạng là: ( 393 - 1): 8 + 1 = 50 (số hạng)
Tổng B có 50 nhóm mỗi nhóm có giá trị là -8
B = -8 \(\times\) 50 = - 400
c,
C = 1 - 2 - 3 + 4 + 5 - 6 +...+ 97 - 98 - 99 +100
C = ( 1 - 2 - 3 + 4) + ( 5 - 6 - 7+ 8) +...+ ( 97 - 98 - 99 + 100)
C = 0 + 0 + 0 +...+0
C = 0
d, D = 22024 - 22023- ... +2 - 1
2D = 22005- 22004 + 22003+...- 2
2D + D = 22005 - 1
3D = 22005 - 1
D = (22005 - 1): 3
Ta có; B= 3100-399+398-397+...+32-3+1
= (3100-399) + (398-397) + ... + (32-3) + 1
= 399 + 397 + ......... + 3 + 1
=> 3B = 3100 + 399 + 397 + ......... + 3
3B - B = 3100 - 1
=> B = \(\frac{3^{100}-1}{2}\)
Ta có; B= 3100-399+398-397+...+32-3+1
= (3100-399) + (398-397) + ... + (32-3) + 1
= 399 + 397 + ......... + 3 + 1
=> 3B = 3100 + 399 + 397 + ......... + 3
3B - B = 3100 - 1
=> B = \(\frac{3^{100}-1}{3}\)