,
1250+90000×0; =14400x+259200; =16000x+256000; =(36+2x)×8000×0;  .9
 ,
 ;
(3)90000÷(6×12)=1250元.5%=1250+450=1700(元);  ;
(2)x=2时;  .5%
=(32+2x)×8000
 .25n+1706;  ,
P=1250+[90000-(n-1)×1250]×0; y
2=(18+12+6+2x)×8000×90%
 .25(n-1)=-6;  ,两种优惠方案的总金额一样多; =1700-6; =1250+450-6.25(n-1)
 
(1)y1=(18+12+1/3×6+2x)×8000