Question

tim so tu nhien n biet rang : 1+2+3+.....+n=1275

Answer 1

1 + 2 + 3 + ... + n = 1275

=> (1 + n).n:2 = 1275

=> (1 + n).n = 1275.2

=> (1 + n).n = 2550

=> (1 + n).n = 51.50

=> n = 50

Answer 2

1 + 2 + 3 + ... + n = 1275

=> (1 + n).n:2 = 1275

=> (1 + n).n = 1275.2

=> (1 + n).n = 2550

=> (1 + n).n = 51.50

=> n = 50