Rotation Transformation - find center

Hi,

This is a rotation transform problem, that I am struggling with.

The vertices of Triangle ABC are A(1,1), B(2, 2), and C(0,3). The vertices of Triangle A1B1C1 are A1(-2, 2), B1(-3,3), and C1(-4, 1). Describe fully the single transformation which maps Triangle ABC onto A1B1C1.

http://i.imgur.com/UjTpX.png

I can make out that it is a rotation of 90 degrees anti-clockwise. But what is the center of the rotation. How do you find the center of rotation for such transformations where the center is not origin. Thanks for your help!

P.S. Ignore the small letters, haven't figured out how to hide those yet!

Re: Rotation Transformation - find center

Sorry for a newcomer to bump an old thread, but my daughter was struggling with just this problem for her homework tonight and Google found this as an answer. The terminology baffled me a bit until she recalled bisecting a line with a compass. We printed the homework page, and used the pencil compass to find the half way lines between the two points. It was brilliant that where they intersected could clearly be seen as the rotation point. I knew there would be a failsafe method of finding it. Earboth - thank you very much - I even get the bit about why it works. :-)