✔ 最佳答案
They are expecting your son to notice a pattern. The sum of the first N odd numbers is always N × N (or as mathematicians would say it, N² meaning N "squared").
1 odd number:
1 = 1 = 1²
2 odd numbers:
1 + 3 = 4 = 2²
3 odd numbers:
1 + 3 + 5 = 9 = 3²
4 odd numbers:
1 + 3 + 5 + 7 = 16 = 4²
So it would seem logical to conclude that if you add the odd numbers from 1 to 29 (that's a total of *15* odd numbers) that the answer would be 15² = 225
1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 + 19 + 21 + 23 + 25 + 27 + 29 = 15² = 225
The following picture should help your son visualize why this makes sense.
1 is a perfect square.
When you add 3 to it, you get a new square that is 2 by 2 = 4.
Then when you add 5 to it, you get a new square that is 3 by 3 = 9.
You add 7 to it to get a square that is 4 by 4 = 16, etc.
So if you add 15 odd numbers (1 + 3 + 5 + ... + 25 + 27 + 29) you could arrange them similar to the diagram below into a square that was 15 by 15 = 225.
Answer:
C) 225
UPDATE:
The only hard part is figuring out how many odd terms you have in your sum. It may not be immediately apparent that there are 15 odd numbers listed. Your son can write them all out (as I did) and you'll see there are 15 terms.
It's clearly not 29 terms because you are leaving out the even numbers. In fact, you are losing nearly half of them. From 1 to 29, there are 14 even and 15 odd numbers. The number of odd numbers is (1 + 29)/2 = 15
Double-check:
1 + 3 + 5 + 7 + 9 + 11 --> (1 + 11) / 2 = 6 terms
1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 --> (1 + 17) / 2 = 9 terms
So clearly:
1 + 3 + 5 + ... + 25 + 27 + 29 --> (1 + 29) / 2 = 15 terms