a. -20+5=-15 

b. 7

c. 35 + (-4)= 31 

d. -129-37+29-63= -100-100 = -200 

e. 217 -315-117+215 =100-100=0 

f. 15:(-3)+24:2 = -5 + 12 = 7 

g. (-5).(-7)-3.(14)= 35 - 42 = -7 

H. (-15)x(-129+29) = -15 x -100 =1500 

i. 13 x (-125+25) =13 x -100 = -1300 

k. 37(-135+35) = 37 x -100 = -3700

\(S=\left(1+3\right)+3^2\left(1+3\right)+...+3^8\left(1+3\right)=4\left(1+...+3^8\right)⋮4\)

\(3S=3+3^2+3^3+...+3^{10}\\ \Rightarrow3S-S=3+3^2+...+3^{10}-1-3-3^2-...-3^9\\ \Rightarrow2S=3^{10}-1\\ \Rightarrow S=\dfrac{3^{10}-1}{2}\)

Ta có \(S=\left(1+3\right)+\left(3^2+3^3\right)+...+\left(3^8+3^9\right)\)

\(S=\left(1+3\right)+3^2\left(1+3\right)+...+3^8\left(1+3\right)\\ S=\left(1+3\right)\left(1+3^2+...+3^8\right)=4\left(1+3^2+...+3^8\right)⋮4\)

A = 3 + 3² + 3³ + 3⁴ + ... + 3²⁰

3A = 3² + 3³ + 3⁴ + 3⁵ + ... + 3²¹

3A - A = (3² + 3³ + 3⁴ + 3⁵ + ... + 3²¹) - (3 + 3² + 3³ + 3⁴ + ... + 3²⁰)

2A = 3²¹ - 3

A = \(\frac{3^{21}-3}{2}\)

Hướng dẫn : nhân 3A rồi cộng A 

\(A=\frac{1}{3}-\frac{1}{3^2}+\frac{1}{3^3}-\frac{1}{3^4}+...+\frac{1}{3^{99}}-\frac{1}{3^{100}}\)

\(\Rightarrow3A=1-\frac{1}{3}+\frac{1}{3^2}-\frac{1}{3^3}+...+\frac{1}{3^{98}}-\frac{1}{3^{99}}\)

\(\Rightarrow3A+A=1-\frac{1}{3}+...+\frac{1}{3^{98}}-\frac{1}{3^{99}}+\frac{1}{3}-\frac{1}{3^2}+...+\frac{1}{3^{99}}-\frac{1}{3^{100}}\)

\(\Rightarrow4A=1-\frac{1}{3^{100}}\)

\(\Rightarrow A=\left(1-\frac{1}{3^{100}}\right)\div4\)

1/1+(-2)+3+(-4)+.....+19+(-20)

=1-2+3-4+.....+19-20

=(1+3+.....+19)-(2+4+.....+20)

={(19+1).[(19-1):2+1]:2}-{(20+2).[(20-2):2+1]:2}

={20.10:2}-{22.10:2}

=10:2.(20-22)

=5.(-2)

=-10

các ý sau bạn làm theo mẫu nhé                                'cô_bé_DuDu'

tính tổng á :

\(A=3+3^2+3^3+3^4+...+3^{20}.\)

\(\Rightarrow3A=3^2+3^3+3^4+...3^{20}+3^{21}\)

\(\Rightarrow3A-A=\left(3^2+3^3+..+3^{21}\right)-\left(3+3^2+....+3^{20}\right)\)

\(\Rightarrow2A=3^{21}-1\)

\(\Rightarrow A=\frac{3^{21}-1}{2}\)

Chứng tỏ bạn nhé 

Dung minh ch k

cho a hỏi là x hay nhân

\(\frac{3^4.5-3^6}{3^4.13+3^4}=\frac{3^4.5-3^4.3^2}{3^4.13+3^4.1}\)

                    \(=\frac{3^4.\left(5-3^2\right)}{3^4.\left(13+1\right)}\)

                    \(=\frac{5-9}{13+1}\)

                    \(=\frac{-4}{14}=\frac{-2}{7}\)

1.3.5...39/21.22.23...40=(1.3.5...39)(2.4.6...40)/(21.22...40)(2.4.6...40)
=1.2.3.4...40/21.22...40(1.2.3...40)2^20
=1/2^20