798X602
=(800-2)X(600+2)
=800X600+800X2-2X600-2X2
=480000+1600-1200-4
=480396拦水坝的横断面是梯形,要建造3AB长的水坝需用多少方土?梯形上底为3A^2-2B,下底为3A+4B,高为2A^2B,帮忙解一下，谢谢了1/2X[(3A^2-2B)+(3A+4B)]X2A^2BX3AB=6A^3B^2X(3A^2+3A+2B)=18A^5B^2+18A^4B^2+12A^3B^3