Hướng dẫn giải:

a) 3 x (20 – 5)

Cách 1:

3 x (20 – 5) = 3 x 15 = 45

Cách 2:

3 x (20 – 5) = 3 x 20 – 3 x 5 = 60 – 15 = 45

b) 20 x (40 – 1)

Cách 1:

20 x (40 – 1) = 20 x 39 = 780

Cách 2:

20 x (40 – 1) = 20 x 40 – 20 x 1 = 800 – 20 = 780

Đặt biểu thức là A, ta có:

\(A=\frac{x^{40}+x^{30}+x^{20}+x^{10}+1}{x^{45}+x^{40}+x^{35}+...+x^{10}+x^5+1}\)

\(\Rightarrow A.x^5=\frac{x^{45}+x^{35}+x^{25}+x^{15}+x^5}{x^{45}+x^{40}+x^{35}+...+x^{10}+x^5+1}\)

\(\Rightarrow A.x^5+A=\frac{x^{45}+x^{40}+x^{35}+x^{25}+x^{15}+x^5+x^{40}+x^{30}+x^{20}+x^{10}+1}{x^{45}+x^{40}+x^{35}+...+x^{10}+x^5+1}\)

\(\Rightarrow A.x^5+1=1\)

\(\Rightarrow A=\frac{1}{x^5+1}\)

\(\frac{x+20}{x-10}=\frac{x+40}{x+70}\)

\(\Rightarrow\left(x+20\right)\left(x+70\right)=\left(x+40\right)\left(x-10\right)\)

\(x^2+20x+70x+1400=x^2+40x-10x-400\)

\(90x+1400=30x-400\)

\(60x=-1800\)

\(x=-30\)

\(x+20+x-40-x=10\)

                      \(x+x-x=10-20+40\)

                                      \(x=30\)

k mk nha 

thank you

x+20+x-40-x=10

x=10+40

x=50

x=50-20

x=30

thử lại:30+20+30-40-30=10

kết mk nha , thank you

đáp án là : 30 nha

ĐKXĐ: \(\hept{\begin{cases}x\ne10\\x\ne-70\end{cases}}\)

\(\frac{x-20}{x-10}=\frac{x+40}{x+70}\)

\(\Rightarrow\left(x-20\right)\left(x+70\right)=\left(x+40\right)\left(x-10\right)\)

\(\Rightarrow x^2+70x-20x-1400=x^2-10x+40x-400\)

\(\Rightarrow20x=1000\Rightarrow x=50\)

                                                 Vậy x = 50

20+x=40+60

\(\Rightarrow\)20+x=100

\(\Rightarrow\)x=100-20

\(\Rightarrow\)x=80

10+90=40+x

\(\Rightarrow\)100=x+40

\(\Rightarrow\)x=100-40

\(\Rightarrow\)x=60

20 + x = 40 + 60

=> 20 + x = 100

=>         x = 100 - 20

=>         x = 80

10 + 90 = 40 + x

=> 100  = 40 + x

=> x = 100 - 40

=> x = 60

20+x=40+60

20+x=100

x=100-20

x=80

X + 8 = 10

X = 10 − 8

X = 2

X + 5 = 20

X = 20 − 5

X = 15

40 + x = 68

X = 68 − 40

X = 28

10/40 = 1/4

x/2 = 1/4 => x.4 = 1.2 => x = 1.2/4 = 0,5

5/y = 1/4 => y.1 = 5.4 => y = 20

z/8 = 1/4 => z.4 = 8.1 => z = 8.1/4 = 2