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POLYGONA  Area of an Irregular Polygon 
This challenge is to calculate the area of an irregular convex polygon. For each test there will be an arbitrarily ordered list of x,y coordinates (integer, 1000 <= x,y <= 1000) for each point in the polygon. For each test there will be N points, where N <= 500.
Input
The first line will contain a single value T for the number of tests, T <= 100.
Each test will begin with a single value N for the number of points in that test, N <= 500. The following N lines will contain the x,y coordinates of the polygon's corners, where x and y are integers in the range 1000 to 1000.
Output
For each test case the output should be the area to one decimal place.
Example
Input: 3 3 0 0 100 0 200 100 4 60 50 40 30 60 50 40 30 5 0 30 50 90 200 0 0 60 400 60 Output: 5000.0 8000.0 21000.0
Added by:  Jander 
Date:  20120422 
Time limit:  5s 
Source limit:  50000B 
Memory limit:  1536MB 
Cluster:  Cube (Intel G860) 
Languages:  All except: ASM64 SCM qobi 
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20160111 17:35:28 Mitch Schwartz
@Dominique Thanks, I didn't see your comment until now. 

20150904 11:22:19 Dominique VAILLANT
@Mitch: POLYGONA, INTER, CONTFRAC, TETRAHE1,... what a great firework in Ruby! 