Problem Solving (PS) | Re: If the sum of all the even numbers from 1 to n(n: odd number) is 69*70

==> If n=2m+1, the even numbers until 2m+1 becomes until 2m, and the sum of the even numbers is 2+4+…+2m=2(1+2+…m)=2m(m+1)/2=m(m+1). From m(m+1)=69*70, you get m=69, which becomes 2m+1=2(69)+1=139.

The answer is C.
Answer: C

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