Asegment with endpoint a(3,1) and b(-1,1) is rotated 90° about the origin. what are the coordinates of a' and b' ? (geometry 10 a lesson 3 unit 5,if you know the answers to this whole practice pls give them to me pls and tysm)

You know that the segment is rotated at a 90 degree angle. 
As seen, the segment is in quadrant one and two. 

For a': 
a(3, 1) --> (-3, -1) 

For b': 
b(-1, 1) --> (1, -1) 

As you can see, the coordinate's signs are reversed for each case.

Hope I helped :) 

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