某商品進價40元/件,當售價為50元/件時,每星期可賣出500件.市場調(diào)查反映,如果每件售價每降1元,每星期可多賣出100件,但售價不能低于42元/件,且每星期至少銷售800件.設(shè)每件降x元(x為正整數(shù)),每星期利潤為y元.
(1)求y與x的函數(shù)關(guān)系式并寫出自變量x的取值范圍;
(2)若某星期利潤為5600元,求商品售價.
(1)依題意,得y=(50-40-x)?(500+100x)=-100x
2+500x+5000,
∵
,
解得3≤x≤8;
(2)當y=5600時,-100x
2+500x+5000=5600,
解得x
1=2(舍去),x
2=3,
∴商品售價為:50-2=48元或50-3=47元.