\(C=1.2+2.3+3.4+...+x.\left(x-1\right)\)

\(\Rightarrow3C=1.2.3+2.3.3+3.4.3+...+x.\left(x-1\right).3\)

\(\Rightarrow3C=1.2.\left(3-0\right)+2.3.\left(4-1\right)+3.4.\left(5-2\right)+...+x.\left(x-1\right).\left[\left(x+1\right)-\left(x-2\right)\right]\)

\(\Rightarrow3C=\left(1.2.3-0.12\right)+\left(2.3.4-1.2.3\right)+\left(3.4.5-2.3.4\right)+...+\left[x.\left(x-1\right)\left(x+1\right)-x.\left(x-1\right)\left(x-2\right)\right]\)

\(\Rightarrow3C=-0.1.2+x.\left(x-1\right)\left(x+1\right)\)

\(\Rightarrow3C=x.\left(x-1\right)\left(x+1\right)\)

\(\Rightarrow C=\dfrac{x.\left(x-1\right)\left(x+1\right)}{3}\)

3C=1x2x3+2x3x3+3x4x3+...+Xx(X+1)=

=1x2x3+2x3x(4-1)+3x4x(5-2)+...+Xx(X+1)[(X+2)-(X-1)]=

=1x2x3-1x2x3+2x3x4-2x3x4+3x4x5-...-(X-1)xXx(X+1)+Xx(X+1)x(X+2)=

=Xx(X+1)(X+2)

\(97-\left(64-x\right)=44\)

\(64-x=97-44\)

\(64-x=53\)

\(x=64-53\)

\(x=11\)

x-57/7=270

x=270+57/7

x=1890/7+57/7

x=1947/7

\(E=\dfrac{3}{1x5}+\dfrac{3}{5x9}+...+\dfrac{3}{121x125}\)

\(\dfrac{4}{3}xE=1-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{9}+...+\dfrac{1}{121}-\dfrac{1}{125}\)

\(\dfrac{4}{3}xE=1-\dfrac{1}{125}\)

\(E=\dfrac{124}{125}x\dfrac{3}{4}=\dfrac{93}{125}\)

2 á má

=2 ạ nhưng mà đừng đăng câu hỏi như này nữa ạ

219 - 7(x + 1) = 10²

7(x + 1) = 219 - 10²

7(x + 1) = 219 - 100

7(x + 1) = 119

x + 1 = 119 : 7

x + 1 = 17

x = 17 - 1

x = 16

--------------------

(3x - 2⁴) . 7³ = 2.7⁴

(3x - 16).7³ = 2.7⁴

3x - 16 = 2.7⁴:7³

3x - 16 = 2.7

3x - 16 = 14

3x = 14 + 16

3x = 30

x = 30 : 3

x = 10

D = \(\dfrac{1}{1.4}\) + \(\dfrac{1}{4.7}\) + \(\dfrac{1}{7.10}\)+...+ \(\dfrac{1}{91.94}\)

D = \(\dfrac{1}{1}\) - \(\dfrac{1}{4}\) + \(\dfrac{1}{4}\) -  \(\dfrac{1}{7}\) +  \(\dfrac{1}{7}\) - \(\dfrac{1}{10}\)+...+ \(\dfrac{1}{91}\) - \(\dfrac{1}{94}\)

D = \(\dfrac{1}{1}\) - \(\dfrac{1}{94}\)

D = \(\dfrac{93}{94}\)

\(10+2\cdot x=4^5:4^3\)

\(\Rightarrow10+2\cdot x=4^{5-3}\)

\(\Rightarrow10+2\cdot x=4^2\)

\(\Rightarrow10+2\cdot x=16\)

\(\Rightarrow2\cdot x=16-10\)

\(\Rightarrow2\cdot x=6\)

\(\Rightarrow x=\dfrac{6}{2}=3\)

__________________

\(2\cdot x-6^2:18=3\cdot2^2\)

\(\Rightarrow2\cdot x-36:18=12\)

\(\Rightarrow2\cdot x-2=12\)

\(\Rightarrow2\cdot x=12+2\)

\(\Rightarrow2\cdot x=14\)

\(\Rightarrow x=\dfrac{14}{2}=7\)

_________________

\(70-5\cdot\left(x-3\right)=3\cdot2\)

\(\Rightarrow70-5\cdot\left(x-3\right)=6\)

\(\Rightarrow70-5x+15=6\)

\(\Rightarrow-5x+15=6-70\)

\(\Rightarrow-5x+15=64\)

\(\Rightarrow-5x=64-15\)

\(\Rightarrow-5x=49\)

\(\Rightarrow x=-\dfrac{49}{5}\)

\(5\cdot4^2-18:3^2\)

\(=5\cdot16-18:9\)

\(=80-2\)

\(=78\)

________________

\(6^2:4\cdot3+2\cdot5^3\)

\(=36:12+2\cdot125\)

\(=3+250\)

\(=253\)

_______________

\(80-\left(4\cdot5^2-3\cdot2^3\right)\)

\(=80-\left(4\cdot25-3\cdot8\right)\)

\(=80-\left(100-24\right)\)

\(=80-76\)

\(=4\)

6² : 4 . 3 + 2 . 5³

= 36 : 4 . 3 + 2 . 125

= 9 . 3 + 250

= 27 + 250

= 277

Hai bài còn lại xem của bạn Phong nhé. Bài này Phong giải chưa chính xác nên Thầy giải lại cho em!