\(\left(4n+3\right)⋮\left(2n+1\right)=>2\left(2n+1\right)+1⋮\left(2n+1\right)\\ =>1⋮\left(2n+1\right)\\ =>2n+1\inƯ\left(1\right)=\left\{\pm1\right\}\\ =>2n\in\left\{0;-2\right\}\\ =>n\in\left\{0;-1\right\}\)

\(\left(n+3\right)⋮\left(n-1\right)=>\left(n-1\right)+4⋮\left(n-1\right)\\ =>4⋮\left(n-1\right)\\ =>n-1\inƯ\left(4\right)=\left\{\pm1;\pm2;\pm4\right\}\\ =>n\in\left\{0;2;-1;3;-3;5\right\}\)

Bán kính hình tròn :

     `130:2=65(cm)`

Diện tích mặt bàn là :

      \(3,14\times65\times65=13266,5\left(cm^2\right)\)

\(\dfrac{x-5}{1990}+\dfrac{x-15}{1980}=\dfrac{x-1990}{5}+\dfrac{x-1980}{15}\\ =>\dfrac{x-5}{1990}-1+\dfrac{x-15}{1980}-1=\dfrac{x-1990}{5}-1+\dfrac{x-1980}{15}-1\\ =>\dfrac{x-1995}{1990}+\dfrac{x-1995}{1980}-\dfrac{x-1995}{5}-\dfrac{x-1995}{15}=0\\ =>\left(x-1995\right).\left(\dfrac{1}{1990}+\dfrac{1}{1980}-\dfrac{1}{5}-\dfrac{1}{15}\right)=0\\ =>x-1995=0\\ =>x=1995\)

`x/3=-5/21`

`=>x=(-5/21).3`

`=>x=-15/21=-5/7`
Vậy : `x=-5/7`