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  1. #include <stdio.h>
  2.  
  3.  
  4. int main() {
  5.  
  6.    // рабочие переменные
  7.    double s = 0;
  8.    int i, n ;
  9.  
  10.    double b, x, xPrev = 0, xPrev2 = 0, xPrev3 = 0;
  11.    int k;
  12.  
  13.    if( scanf("%d", &n) <= 0 )
  14.    {
  15.       printf("error: missing n\n" );
  16.       return 0;
  17.    }
  18.  
  19.    if( n <= 0 )
  20.    {
  21.       printf("error: out of range: n=%d\n", n);
  22.       return 0;
  23.    }
  24.  
  25.    // основной цикл сравнения   
  26.    for( i = 1, b = 2; i <= n; i++, b *= 2 )
  27.    {
  28.       if( i > 3 ) xPrev3 = xPrev2;
  29.       if( i > 2 ) xPrev2 = xPrev;   
  30.       if( i > 1 ) xPrev = x;  
  31.  
  32.       if( i < 4 )
  33.       {
  34.         x = 1;
  35.       }
  36.       else
  37.       {
  38.         x = xPrev + xPrev3;
  39.       }
  40.  
  41.       s += x / b;
  42.    }
  43.  
  44.    // вывод результата
  45.    printf("result: %lf\n", s );
  46.    return 0;
  47. }
Success #stdin #stdout 0s 3300KB
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result: 1.000000
https://ideone.com/Yy8e5l
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
secret

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