Easiest Method — Coefficient of \( x^{50} \)

Given Expression:

\[ (1 + x)^{1000} + 2x(1 + x)^{999} + 3x^2(1 + x)^{998} + \cdots + 1001x^{1000} \]

This follows a known identity that simplifies the full expression to:

\[ f(x) = (1 + x)^{1002} \]

Now: The coefficient of \( x^{50} \) in \( f(x) \) is:

\[ \boxed{\binom{1002}{50}} \]

✅ Final Answer:   \( \boxed{\binom{1002}{50}} \)