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题库 / OG2017Q-DS-267
If x2 + y2 = 29, what is the value of (x - y)2 ?
(1) xy = 10
(2) x = 5
Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
EACH statement ALONE is sufficient.
Statements (1) and (2) TOGETHER are NOT sufficient.
题目分析：
本题求~$(x-y)^2$~，展开得~$x^2+2xy+y^2$~。
（1） 已知xy=10和~$x^2+y^2=29$~，所以~$x^2+2xy+y^2=10*2+29=49$~，题目得解。
（2） 仅知道x=5，不能确定y的值，也就不能计算~$(x-y)^2$~的值。
因此，本题答案为（A），仅条件1可以解题，条件2不行。
(x-y)^2 = x^2-2xy+y^2
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