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Yizhi mathematics
Branch 1: Story books start with odd pages, with at least 1 page!

As for the most need to calculate.

The first story begins with a 1= 1.

The opening page of the second story is a2=a 1+ 1=2.

The third one is A3 = A2+2 = A1+1+2 =1+1+2 = 4.

The fourth a4 = a3+3 =1+1+2+3 = 7.

……

The nth story begins with an = a (n-1)+(n-1) =1+1+2+...+(n-1) =1+(.

A70= 1+69*70/2 is an even number.

A69= 1+68*69/2 is an odd number 2347.