Question

aratati ca nr. 9x(5+10+15+....+200):41este patrat perfect

Answer 1

9x(5+10+15+....+200):41=
=9x5(1+2+3+....+40):41=
=9x5x40(40+1):2 :41=
=9x5x40:2 =
=900=30²

Answer 2

 $9\cdot5*(1+2+3+...+40):41=9\cdot5\cdot\frac{4o\cdot\not41}{2}:\not41$=9*5*2o=(3*10)²=900.