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9 ori(5plus 10plus 15plus...plus200):41este patrat perfect

Răspuns :

In primul rand suma (5+10+15+........200)=5x(1+2+3+.........40)=5x40x(40+1):2= 5x40x41:2=5x20x41=4100
Deci rezul ca numarul=9x4100:41=900
900=30^2⇒deci 30^2 este patrat perfect