Question

Arata ca numarul n= 1+2+....+20+75÷5 este pătrat perfect

Answer 1

n = 1 + 2 + .. + 20 + 75 : 5

n = 20·21 : 2 + 15

n = 210 + 15

n = 225

n = 15²

Answer 2

n = 1 + 2 + 3 + ...... + 20 + 75 : 5

n = 20 × ( 1 + 20 ) / 2 + 15

n = 10 × 21 + 15

n = 210 + 15

n = 225

n = 15² → patrat perfect

225 = 15 x 15