Method 1: Deduction algorithm-Because * * * has 20 groups of numbers, the tenth and eleventh items are just the middle two groups of numbers, then the sum of the ninth and twelfth items is also 50, and so on, the sum of the first and twentieth items is also 50, that is, 50 * 20/2 = 500.

Method 2:

(a 1+9d)+(a 1+ 10d)= 50

Get 2*A 1+ 19d=50.

Sum formula: x = (a1+an) * n/2 = {a1+[a1+(20-1) d]} * 20/2 = {2 * a/kloc-

=50* 10=500