Question 860385
to find the 20th term 
a20=20^2=400
The sum of the first 25 terms is the formula given
n/6(n+1)(2n+1) where n =25
25/6(25+1)(2*25+1)
25/6*26*51
25/(2*3)*2*13*3*17

25*13*17=5525