已知:(A+B)=X,(A-B)=Y.A²+B²=[(A+B)²+(A-B)²]÷2=(x²+y²)/2;AB=[(A+B)²-(A-B)²]÷4=(x²-y²)/4;求2分之(A的平方+B的平方)+AB,=(x²+y²)/4+(x²-y
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