Question

[Nguyễn Ngân Hà]

Tìm x thuộc N, biết:

1 + 2 + 3 + ... + x = 78

Answer 1

[The love of Shinichi and Ran is imm...]

1+2+3+...+x=78

=>[x(x+1)]:2=78

x(x+1)=146

x(x+1)=2².3.13

x(x+1)=12.13

=>x=12

Answer 2

[DORAEMON]

78=1+2+3+4+5+..+12

X=12