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The square of (a plus b) (a minus b) plus (a plus b) minus 2A square
=A square - b square + a square + b square + 2ab-2a square
=2ab
As shown in the figure, CD is the two points on the line AB, e is the midpoint of AC, f is the midpoint of BD, if EF = 18, CD = 6, the length of AB is
Ef-cd = EC + DF = AE + BF = 18-6 = 12, so AB = EF + AE + BF = 12 + 18 = 30
As shown in the figure, it is known that the line segment ad = 6cm, the line segment AC = BD = 4cm, e and F are the midpoint of the line segments AB and CD respectively, and EF is calculated
∵AD=6cm，AC=BD=4cm，∴BC=AC+BD-AD=2cm；∴EF=BC+12（AB+CD）=2+12×4=4cm．
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