NOJ [1132] Evil Wheel

• 问题描述
• The Demon is coming to our worlds. He's using a wheel with a thorn on the border.
  The wall where the thorn through will be separated from the wall.
  

  Now you need to know the weight of the separated wall when the wheel turns a circle. You just think the thorn is on the very bottom of the wheel.

  
  
• 输入
• This problem has several cases. Input until EOF.
  Each case contains 2 decimals: R and W, Indicate the radius of the wheel and the weight of unit area of the wall. (0 < R, W < 100)
  
• 输出
• For each case, you should print the whole weight of the separated wall. Keep six decimal places.
  
  
  
  
  积分求面积，不懂的去看高数书
  
  

  #include<stdio.h> 
  #define PI 3.1415926535897932384626433 
   
  int main()
  { 
   double R,W;
   while(~scanf("%lf%lf",&R,&W))
   {
   printf("%f\n",3*PI*R*R*W); 
   }
   return 0;
  }