Răspunsuri

2014-07-26T10:47:33+03:00
9*5(1+2+....+40):41=9*5*40*41:2:41=45*40:2=900=30^2 ⇒ este patrat perfect
am folosit formula 1+2+3+...n=n(n+1):2 unde n=40
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