因为a+b=2
(a+b)^2=2^2=4
a^2+2ab+b^2=4 (1)
把ab=1代入（1）得：
a^2+b^2=4-2=2
(a-b)^2=a^2-2ab+b^2=2-2*1=0
a的平方+b的平方=(a+b)²-2ab=4-2=2
(a-b)²=(a+b)²-4ab=4-4=0
a²+b²
=(a+b)²-2ab
=4-2
=2
(a-b)²
=a²+b²-2ab
=2-2
=0
a-b=0
a + b = 2
ab = 1
a² + b² = (a + b)² - 2ab = 4 - 2 = 2
(a - b)² = (a + b)² - 4ab = 4 - 4 = 0
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