# math

posted by M

The 20 leaves of a 40-page book are numbered 1 through 40. The first and last leaves are ripped out. What is the sum of the remaining page numbers?

1. bobpursley

So what is the sum of 3+4+5+...37+38 ?

It is an arithemetic sum, first term 3, d=1
http://en.wikipedia.org/wiki/Arithmetic_progression

n= 36 terms

Sum= n/2 (2a+(n-1)d)

2. SK

I got 738

3. Milowen

738

Using the formula of an Arithmetic Progression, we can solve the answer.

Since 1+2+3+4+...+37+38+39+40 = 41*20 = 820,
the answer is 820-40-39-2-1 = 738.

(If there are N pages in a book, there are N/2 leaves)

Hope I helped!

Milowen

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