题目

已知A=B=R,x∈A,y∈B,f:x→y=ax+b,若5和20的原象分别是5和10,则7在f下的象f(7)=. 答案：11解析:由解得故f(7)=7a+b=11.13、You’d better voice your opinion publicly     you feel it the best time for it at the meeting. A．that B．until    C．when D．where