Răspunsuri

2014-08-05T18:07:14+03:00
9*5(1+2+3+....+40)/41=9*5*40*41/2*41=9*100=900=30²
am folosit suma lui Gauss 
1+2+3+....+40=40*41/2
2014-08-05T18:13:58+03:00
5+10+15+...200=(5+200)40/2=205*20=4100
 
=> 9*4100/41=900 (patrat perfect)