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# can you help me solve this maths problem for my son? The answer is C:225 but can't figure out how they arrived at that?

### 13 Answers

- PuzzlingLv 71 month agoFavorite Answer
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

- KrishnamurthyLv 71 month ago
1 + 3 + 5 + 7 + ...... + 25 + 27 + 29

sum_(n=1)^15(2 n - 1) = 225

Answer choice:

C. 225

- AmyLv 71 month ago
Look at the pattern. On the left side they are adding the first N odd numbers:

1 (one odd number)

1 + 3 (two odd numbers)

1 + 3 + 5 (three odd numbers)

1 + 3 + 5 + 7 (four odd numbers)

On the right is N^2.

You can do the addition yourself to confirm that these are indeed equal.

They want to know what 1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 + 19 + 21 + 23 + 25 + 27 + 29 equals. That's the first 15 odd numbers, so the answer is 15^2.

A simple way to prove this sum is to pair the odd numbers and replace them.

1 + 29 = 30 = 15 + 15

3 + 27 = 30 = 15 + 15

5 + 25 = 30 = 15 + 15

...

therefore 1 + 3 + 5 ... + 25 + 27 + 30 = 15 + 15 + 15 ... + 15 + 15 + 15.

The sum of 15 15's is 15^2.

- L. E. GantLv 71 month ago
Essentially, you are adding up the odd numbers...

so count the number of numbers: 15

using induction and using S_n to represent the sun of the first n numbers in the series:

S_1 = 1 = 1*1 ..... n*n where n = 1

S_2 = 1+3 = 2*2.... n*n where n = 2

S_3 1+3+5 = 3*3....

so S_15 = 15*15 = 225

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- JOHNLv 71 month ago
How many numbers in the sequence 1, 2, 3,....,29? There are 15 in all. You could list them all count them. Or you could argue they are all the numbers from 1 to 29 less the even numbers, which are 2, 4, 6,.....,28. The latter amount to 28 ÷ 2, = 14, so the odd ones amount to 29 -14 =15. The pattern suggests they add up to 15 x15 = 225 (1 = 1 x 1, 1 + 3 = 2 x 2, 1 + 3 + 5 = 3 x 3, 1 + 3 + 5 + 7 = 4 x 4).

- ignoramusLv 71 month ago
He will be expected to use the pattern shown by the given examples to deduce a method of finding the sum of any such sequence (not to use fancy equations which he may not yet have been taught).

1 = 1 x 1

1 + 3 = 2 x 2

1 + 3 + 5 = 3 x 3

1 + 3 + 5 + 7 = 4 x 4

Look for the pattern : where does the (squared) number on the right-hand side come from ?

The first line is trivial, and gives no useful information. The second line gives 1 + 3 = (4), and a half of 4 = 2

The third line : 1 + 5 = (6), half of 6 = 3

Fourth line : 1 + 7 = (8), half of 8 = 4

So, it should be clear now : you add the first number and the last number, take half of it, and square it.

Hence the solution to the given problem is 1 + 29 = (30), half of which is 15, hence the sum is 15 x 15.

(And notice that the first line still fits into the pattern : 1 is both the first number AND the last number, so 1 + 1 = 2, and half of 2 =1 !)

- DavidLv 71 month ago
The nth term is 2n-1 and the missing numbers are 9+11+13+15+17+19+21+23 and they all add up to 225 which is option C

- lenpol7Lv 71 month ago
Use the equation

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

Where

n = is the number of terms ( 15 in this case)

a = starting value (1 in this case)

d = difference between each term ( 2 in this case)

Substituting

S(15) = (15/2)(2(1) + ( 15 - 1)2))

S(15) = (15/2)(2 + 14(2))

S(15) = (15/2)(2 + 28)

S(15) = (15/2)(30)

Cancel down by '2'

S(15) = 15(15)

S(15) = 225 As required!!!!

Hope that helps!!!!

- Robert JLv 71 month ago
The "pattern below" shows that the sum of a series of odd numbers starting at 1 is equal to the square of the midpoint of that series.

Add the first and last digits of such a series, divide by two, then square the result.

The midpoint of the series given (1 to 29) is 15.15^2 = 225

You can cross-check by adding all the odd numbers from 1 to 29. The total is 225.