Finding the moment of inertia for a specific figure (illustrated)

[PhyIsOhSoHard]

Homework Statement

http://img577.imageshack.us/img577/2069/inertia.gif

The following figure with these known lengths are given. I need to find the moment of inertia.

Homework Equations

According to my book's facit, this is the solution:
http://img845.imageshack.us/img845/6725/inertiaformula.png

The Attempt at a Solution

This is what I understand so far...
The fraction h/2 refers to half the figure (when cut from horizontal). So you start from the middle of the figure, and then move up to the end in which case you have traveled the vertical length h/2. Then by multiplying the entire integral with 2, you take the bottom half of the figure into account as well (since it's symmetrical).

I have no idea about the fraction. I've tried, but I have no idea how they came up with that. All I know is that "y" is the vertical axis for the figure...

 

[tiny-tim]

Welcome to PF!

Hi PhyIsOhSoHard! Welcome to PF!

I have no idea about the fraction. I've tried, but I have no idea how they came up with that. All I know is that "y" is the vertical axis for the figure...

it's similar triangles …

you want the base of the smaller triangle, and you know that the base of the larger triangle is b