Look at the numbers 1, 3, 6, 10, 15, 21, 28, 1 According to the law, what is the number from one to the hundredth? What is the number in 2009?

Look at the numbers 1, 3, 6, 10, 15, 21, 28, 1 According to the law, what is the number from one to the hundredth? What is the number in 2009?

an=n(n+1)/2
a100=100*101/2=5050
a2009=2009*2010/2=2019045