输出3个结果，分别是0.000000和1.000000和1.500000
f(0)=1;f(1)=2;f(2)=2.5;a=0a=a+f(0)=1.000000a=a+f(1)=1+2=3.000000a=a+f(2)=3+2.5=5.500000
