括号2/3加分之一括号乘以括号2/3减1/2括号,简便算法。括号2/3加分之一括号乘以括号2/3减1/2括号=（2/3+1/2）*(2/3-1/2)=(2/3)^2-(1/2)^2=4/9-1/4=16/36-9/36=7/36
