21=20+1=x+1

Thay 20=x+1 vào biểu thức rồi triệt tiêu

câu sau tương tự

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4x² - 20x + 9 = 0

(4x^2 - 2x) - (18x - 9) = 0

2x(2x - 1) - 9(2x - 1) = 0

(2x - 1)(2x - 9) = 0

2x - 1 = 0 hoặc 2x - 9 = 0

x = 1/2 hoặc x = 9/2

4x² - 20x + 9 = 0

(4x^2 - 2x) - (18x - 9) = 0

2x(2x - 1) - 9(2x - 1) = 0

(2x - 1)(2x - 9) = 0

2x - 1 = 0 hoặc 2x - 9 = 0

x = 1/2 hoặc x = 9/2

\(a,\Rightarrow4x^2+20x+25=0\Rightarrow\left(2x+5\right)^2=0\Rightarrow2x+5=0\Rightarrow x=-\dfrac{5}{2}\\ b,\Rightarrow x^3-6x^2+12x-8=0\Rightarrow\left(x-2\right)^3=0\Rightarrow x-2=0\Rightarrow x=2\)

a) \(\Rightarrow4x^2+20x+25=0\)

\(\Rightarrow\left(2x+5\right)^2=0\Rightarrow2x+5=0\)

\(\Rightarrow x=-\dfrac{5}{2}\)

b) \(\Rightarrow x^3-6x^2+12x-8=0\)

\(\Rightarrow\left(x-2\right)^3=0\Rightarrow x=2\)

a. `4x^2-20x+25=0`

`<=>(2x)^2-2.2x.5 +5^2=0`

`<=>(2x-5)^2=0`

`<=>2x-5=0`

`<=>x=5/2`

b. `(x-5)(x+5)-(x-3)^2=2(x-7)`

`<=>x^2-25-x^2+6x-9=2x-14`

`<=>6x-34=2x-14`

`<=>4x=20`

`<=>x=5`

\(a,4x^2-20x+25=0\Leftrightarrow\left(2x\right)^2-2.2x.5+5^2=0\)

\(\Leftrightarrow\left(2x-5\right)^2=0\Leftrightarrow x=\dfrac{5}{2}\)

b, \(\left(x-5\right)\left(x+5\right)-\left(x-3\right)^2=2\left(x-7\right)\)

\(\Leftrightarrow x^2-25-x^2+6x-9=2x-14\Leftrightarrow4x=20\Leftrightarrow x=5\)

a) Có: (2x)² - 2.2.5.x + 5² = 0

          ⇒ (2x - 5)² = 0 ⇒ 2x - 5 = 0

          ⇒ 2x = 5 ⇒ x = \(\dfrac{5}{2}\)

b) Có: x² - 25 - x² + 6x - 9 = 2x - 14

           ⇒ 6x - 36 = 2x - 14

           ⇒ 4x = 22

           ⇒ x = \(\dfrac{11}{2}\)

Ta có: \(x=-24\Leftrightarrow-x=24\Leftrightarrow1-x=25\)

Thay vào E ta được:

\(E=x^{20}+\left(1-x\right)x^{19}+\left(1-x\right)x^{18}+...+\left(1-x\right)x^2+\left(1-x\right)x+\left(1-x\right)\)

\(E=x^{20}+x^{19}-x^{20}+x^{18}-x^{19}+...+x^2-x^3+x-x^2+1-x\)

\(E=1\)