It is known that 1+2+3+4+5+...+19+20=(1+20)*20/2=210, according to this rule, please calculate the sum of the following formula: a+2a+ How much is 3a+4a+...+99a+100a=, and please express the formula of this summation in words.
已知1+2+3+4+5+…+19+20=(1+20)*20/2=210，根据这个规律，请你算一算下面的式子的和：a+2a+3a+4a+…+99a+100a=多少，并请用文字表达出这种求和式子的公式．
<result>5_050</result>