〔(a+b/2)²+(a-b/2)²)]乘以(2a的平方-1/2b²)
=(2a²+1/2b²)(2a²-1/2b²)
=(2a²+1/2b²)(4a²-b²)/2
=(2a²+1/2b²)(2a+b)(2a-b)/2