2100.32 – Sums of Odd Numbers
Look at the figure:
- What does A have to do with B?
- Continue both A and B for two more "parts".
- Find the totals in A. What kind of numbers are they?
- Does B tell you why these numbers are what they are? Explain.
- What is 1 + 3 + 5 + 7 + . . . + 99?
Solution
- You can find each number in the picture.
- 1 + 3 + 5 + 7 + 9; 1 + 3 + 5 + 7 + 9 + 11
- 1, 4, 9, 16, 25, 36: perfect squares.
- You can see the squares!
- Let's work up to it:
1+3=41+3+5=91+3+5+7=16=22=32=(21+5)2=42=(21+7)2
So 1+3+5+7+…+99=(21+99)2=502=2500