[(a+1/2b)²+(a-1/2b)²]·（2a²-1/2b²）=(a²+ab+1/4b²+a²-ab+1/4b²)(2a²-1/2b²)=(2a²+1/2b²)(2a²-1/2b²)=(18+8)(18-8)=26×10=260
