Tính $A=13.15+15.17+17.19+...+2015.2017$
sao anh toàn đăng bài lớp 6 vậy ? –  Hàn Thiên Dii 15-05-16 08:43 PM
 $A=13.15+15.17+17.19+...+2015.2017$
$=14^2-1+16^2-1+18^2-1+....+2016^2-1$
$=(14^2+16^2+18^2+...+2016^2)-1002$
$=4(7^2+8^2+9^2+....+1008^2)-1002$
$=(4.1008.1009.2017)/6-(4.6.7.13)/6-1002$
$=1367621450 $

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