1/(1/20+1/21+1/22+1/23+...+1/38+1/39) What's the integral part of?

1/(1/20+1/21+1/22+1/23+...+1/38+1/39) What's the integral part of?

double c = 20;
double sum = 0;
for (int i = 0; i < 19; i++)
{
sum += 1 / (c + i);
}
Console.WriteLine (1/sum);
one point four seven zero two three seven two zero two eight six one three three
It should be 1

What is the result of three 2002 digit operations 99.9 × 88.8 / 66.6? What are the characteristics? How to calculate?

99.9=3×33.3
88.8=2×44.4
66.6=2×33.3
Original formula = 3 × 33.3 × 2 × 44.4 / (2 × 33.3)
33.3 × 2 in the middle of the molecule and the denominator disappear
Original formula = 3 × 44.4 = 133... 32
2003, middle 2001, 3

1989×1999-1988×2000．

1989×1999-1988×2000=1989×（2000-1）-1988×2000，=1989×2000-1989×1-1988×2000，=（1989-1988）×2000-1989，=1×2000-1989，=2000-1989，=11．