化简求值：（X-1/2Y-1）（X-1/2Y+1）-（X-1/2Y-1）平方,其中X=1.7,Y=3.9

（X-1/2Y-1）（X-1/2Y+1）-（X-1/2Y-1）平方
=[(X-1/2Y)-1][(X-1/2Y)+1]-[(X-1/2Y)-1]平方
=[(X-1/2Y)平方-1]-[(X-1/2Y)平方-2(X-1/2Y)+1](左边是平方差公式,右边是完全平方公式)
=(X-1/2Y)平方-1-(X-1/2Y)平方+2(X-1/2Y)-1
=-1+2(X-1/2Y)-1
=2X-Y-2
把X=1。7,Y=3。9代入,得
原式=2×1。7-3。9-2=-2。5