4 + 8 + 12 + .. + 4n = 180

=> 4 x ( 1 + 2 + 3 + ... + n ) = 4 x 45

=> 1 + 2 + 3 + .. + n = 45

=> \(\dfrac{\left(n-1\right):1+1}{2}\times\left(n+1\right)\) = 45

=> \(\dfrac{n}{2}\times\left(n+1\right)\) = 45

=> \(\dfrac{n\left(n+1\right)}{2}=45\)

=> \(n\left(n+1\right)=90\)

=> n = 9

Vậy n = 9

       4 + 8 + 12 + ... + 4 * x = 180

 =>  4. ( 1 + 2 + 3 + ... + x) = 180

 =>     1 + 2 + 3 + ... + x     = 180 : 4

 =>      1 + 2 + 3 + ... + x    = 45

 =>     x * ( x + 1 ) : 2          = 45

 =>        x * ( x + 1 )           =  45 * 2 = 90

 =>         x * ( x + 1 )          =   9 * 10

 =>                 x                 =       9. 

      Nếu thấy đúng cho mình xin nha

Trả lời

Hình như đề lỗi thì phải

Mk sẽ gạch chân nhé !

4+8+12+...+4.x=180

\(4+8+12+...+4x=180\)

\(\Rightarrow4.\left(1+2+3+...+x\right)=180\)

\(\Rightarrow1+2+3+...+x=180:4=45\)

\(\Rightarrow x=\left(x+1\right).x:2=45\)

\(\Rightarrow\left(x+1\right).x=45.2=90\Rightarrow90=9.10\)

\(\Rightarrow x=9\)

A = 1/4 x 8 + 1/8 x 12 + 1/12 x 16 + ... + 1/176 x 180

=> 4A = 4/4 x 8 + 4/8 x 12 + 4/12 x 16 + ... + 4/176 x 180

=> 4A = 1/4 - 1/8 + 1/8 - 1/12 + 1/12 - 1/16 + ... 1/176 - 1/180

=> 4A = 1/4 - 1/180

=> 4A = 45/180 - 1/180

=> 4A = 44/180

=> 4A = 11/45

=> A = 11/45 : 4

=> A  = 11/180

Vậy A = 11/180

A = \(\dfrac{1}{4\times8}\) + \(\dfrac{1}{8\times12}\) + \(\dfrac{1}{12\times16}\) +...+ \(\dfrac{1}{176\times180}\)

A = \(\dfrac{1}{4}\) \(\times\)( \(\dfrac{4}{4\times8}\)+ \(\dfrac{4}{12\times16}\)+...+ \(\dfrac{4}{176\times180}\))

A = \(\dfrac{1}{4}\) \(\times\)( \(\dfrac{1}{4}\) - \(\dfrac{1}{8}\) + \(\dfrac{1}{12}\) - \(\dfrac{1}{16}\) +...+ \(\dfrac{1}{176}\) - \(\dfrac{1}{180}\))

A = \(\dfrac{1}{4}\) \(\times\)(\(\dfrac{1}{4}\) - \(\dfrac{1}{180}\))

A = \(\dfrac{1}{4}\) \(\times\)\(\dfrac{11}{45}\)

A = \(\dfrac{11}{180}\)

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Em con quá non