DEFINTION3 函数 当B= Real set Let f and f be function from a to r Then f,+f and f, f, are also functions from a to r defined by (f1+f2)(x)=f1(x)+f2(x) (f1f2)(x)=f1(x)f2(x)F u n c t i o n s 函 数 DEFINITION 3. Let f1 and f2 be function from A to R. Then f1 + f2 and f1 f2 are also functions from A to R defined by (f1 + f2 )(x) = f1 (x) + f2 (x), (f1 f2 )(x) = f1 (x) f2 (x). 当B=Real Set