Question

cho cac day sau :1,3,6,10,15...tim phan tu thu 108 cua day tren

Answer 1

1=1

3=1+2

6=1+2+3

10=1+2+3+4

15=1+2+3+4+5

.A=1+2+..107+108..

=(108+1)/2*108=109*54

Answer 2

1 = 1

3 = 1 + 2

6 = 1 + 2 + 3

10 = 1 + 2 + 3 + 4

15 = 1 + 2 + 3 + 4 + 5

 A = 1 + 2 + ...+ 107 + 108

    = (108 + 1) : 2 x 108 = 109 x 54