Typically, when people contract an infectious disease, their immune system immediately begins to produce extra white blood cells to fight the disease. The scatterplot shows the white blood cells reproducing in an infected patient, along with several values found when modeling the data using a graphing calculator. According to this model, how many white blood cells per microliter of blood did the patient have before he contracted the disease?
分析：问题是：contract the disease 前对应的white blood cells per microliter of blood数量，所以应该求t = 0 时Blood cell数量。图表反应的是：感染病人体内white blood cells同hour的变化关系，呈现上升趋势，并且纵坐标单位为thousands per mcl。结合上升趋势以及table中给出数据判断出为exponential growth model，每过一个小时白细胞数量变成原来两倍。此题特殊之处在于原点并非t = 0 时对应的初始值，所以还要往前推。t = 4, y = 136，所以t = 3, y = 68依次推得：t = 0 , y = 8.5 结合纵轴单位度量，答案为8500。
An occupational health organization published a study showing an increase in the number of injuries that resulted from elderly people falling in the bathtub. In response to this increase, a medical supply company decided to drop its price on bathtub lifts from $450 to $375, hoping to still break even on the lifts. The company breaks even when its total revenue (income from selling n bathtub lifts) is equal to its total cost of producing the lifts. If the cost C, in dollars, of producing the lifts is C = 225 n + 3,150, how many more of the lifts does the company need to sell at the new price to break even than at the old price?
(A) 7 (B) 12 (C) 14 (D) 21
分析：观察问题： how many more of the lifts does the company need to sell at the new price to break even than at the old price?所以是一题减法题。涉及到一个经济学词汇“break even”，向前扫读有解释：total revenue (income from selling n bathtub lifts) is equal to its total cost of producing the lifts 所以意思是收入 = 成本，所以问题问的是新价格下盈亏相抵要比旧价格下盈亏相抵多卖多少电梯。 成本公式为 C = 225 n + 3,150
450a = 225 a + 3,150 375b = 225 b + 3,150 解得： a = 14, b= 21
最终答案为a – b = 7 选 A