(a+1/2b)(a-1/2b)-(3a-2b)(2b+3a)
理解为如下式子:
=[a+(1/2)b][a-(1/2)b]-(3a-2b)(2b+3a)
=(4a²-b²)/4-(9a²-4b²)
=a²-b²/4-9a²+4b²
=-8a²+15b²/4