# perfect squares

• Dec 12th 2009, 03:51 PM
sri340
perfect squares
How many integers between 200 and 300 are perfect squares?
• Dec 12th 2009, 04:25 PM
Gusbob
We know 10 x 10 = 100.

Working from there:

11 x 11 = 121
12 x 12 = 144
13x 13 = 169
14 x 14 = 196

15x15 = 225 but that's over 200.
So there are four perfect squares between 100 and 200 (not inclusive of 100)
• Dec 13th 2009, 02:23 AM
aidan
Quote:

Originally Posted by sri340
How many integers between 200 and 300 are perfect squares?

Essentially, the same idea as Gusbob:

$\sqrt{300} = 17.32...$

$\sqrt{200} = 14.14...$

How many integers between those two numbers?
• Dec 13th 2009, 04:20 AM
Bacterius
You can also use a progression.
Say $T_n = n^2$, (note difference $D_n = (n + 1)^2 - n^2 = n^2 + 2n + 1 - n^2 = 2n + 1$).
How many terms between $200$ and $300$ ? :)

Start from the biggest square under $200$, which is $14^2 = 196$. Now many $D_n$ with $n > 14$ can you fit in $300 - 196 = 104$ ?

Is there any sum of terms of $2n + 1$ with $n > 14$ and starting from $15$ consecutive that fit into $104$ ? Sure ! We have $31 + 33 + 35 < 104$, but we cannot fit $37$, so there are only three perfect squares between $200$ and $300$.

There ... must be a better way to explain this, though ..

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I know this solution is a bit useless and boring after the previous answers, but eh, this is worth a post :D