satOG第三版p90612p9081912.ifyisdirectlyproportionaltox^2andy=1/8whenx=1/2,whatisthepositivevalueofxwheny=9/2?19.thecostofmaintenanceofanautomobileincreaseseachyearby10percent,andAndrewpaid300thisyearform

Question

sat OG 第三版 p906 12 p908 19
12.if y is directly proportional to x^2 and y=1/8 when x= 1/2,what is the positive value of x when y=9/2?
19.the cost of maintenance of an automobile increases each year by 10 percent,and Andrew paid 300 this year for maintenance on his automobile.If the cost c for maintenance on Andrew's automobile n years from now is given by the function c(n)=300x^n,what is the value of

Answer 1

12.题目意思是y与x^2 成比例,既是y=k*x^2 且当x= 1/2,y=1/8 ,将这组解代入y=k*x^2,求得k= 1/2,既是y=x^2/2,又因为当 y=9/2代人y=x^2/2,求得x1=3 或x2=-3
19.题目说每年增加10%成本而且第一年花费300,如果花费c跟某n年有这样关系c(n)=300x^n,求x
300x^n=300（1+10%）^n
解得x=1.1