Question

sat OG 数学题 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 x?

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
解答：300x^n=300（1+10%）^n
解得x=1.1