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Math problems in grade one (about triangles, Shanghai Education Press 7, 14. 1( 1))?
Because A, B and C are three sides of a triangle, the sum of the two sides is greater than the third side, and the difference between the two sides is less than the third side.

So a-b-c < 0 a+b-c > 0.

So |a-b-c|+|a+b-c|

=-(a-b-c)+a+b-c

=-a+b+c+a+b-c

=2b