double s = 0;
       Console.WriteLine("Введите а:");
       double a = Convert.ToDouble(Console.ReadLine());
       Console.WriteLine("Введите b:");
       double b = Convert.ToDouble(Console.ReadLine());
       Console.WriteLine("Введите n:");
       double n = Convert.ToDouble(Console.ReadLine());
       
      double h = (b - a)/n;
      for(int i = 0; i <= n - 1; i++){
          a += i*h;
          s += Math.Cos(1.5) * Math.PI + 0.5 * a;
      }
      double res = s * h;
      Console.WriteLine($"Ответ: {res}");
      
      Console.ReadKey();

Line 2: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
Line 3: error: 'Convert' undeclared here (not in a function)
Line 3: error: 'Console' undeclared here (not in a function)
Line 4: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
Line 6: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
Line 9: error: initializer element is not constant
Line 10: error: expected identifier or '(' before 'for'
Line 10: error: expected '=', ',', ';', 'asm' or '__attribute__' before '<=' token
Line 10: error: expected '=', ',', ';', 'asm' or '__attribute__' before '++' token
Line 14: error: initializer element is not constant
Line 15: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
Line 17: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token