It is known that a, B and C are the three sides of △ ABC. It is proved that | a + B-C | + | a-b-c | = 2B Such as the title

It is known that a, B and C are the three sides of △ ABC. It is proved that | a + B-C | + | a-b-c | = 2B Such as the title

I'll do it
a+b-c-a+b+c=2b
O(∩_ ∩)O~