Determine which of the following two numbers is larger, and explain your reasoning:

a.1,000,000x(1+2+3+4+...+1,000,001)

b.1,000,001x(1+2+3+4+...+1,000,000)

I've tried multiplying the number (1,000,000) times the corresponding numbers (1,2,3,4) and did the same for 1,000,001. However, I'm not sure if I have done that right...