题目

.当a=4,b=27时,求下列各式的值. (1)+; (2)÷(). 答案：解:(1)因为====. 又因为=, 所以原式=+, 故当a=4,b=27时,原式=+2=+(33=+9=. (2)因为原式=÷()=÷(·=b÷(ab)=. 所以原式==(22=.Helen studied hard last term and she got excellent_________ in her exams.A. gifts B. rules C. messages D. grades