b - a
= (500-454)-(500-445)
= 500 -445 -500 +454
= -1
sai a-b =500-(454-445) =500-9 =491
nhầm ,a-b
Thay a và b vào biểu thức a-b, ta có:
(500-445)-(500-454)
=500-445-500+454
=(500-500)+(454-445)
=9
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