两天后考试，请牛人帮我解答三个prep问题，网上搜过，居然没人回答过

这是三道prep-ps 上的题目。分别为ps1的17，63，177题

17，In a certain board game,a stack of 48 cards,8 of which represent shares of stock,are shuffled and then placed face down. If the first 2 cards selected do not represent shares of stock, what is the probability that the third card selected will represent a share of stock?

A1/8

B1/6

C1/5

D3/23

E4/23

63.For which of the following function is f(a+b)=f(a)+f(b) for all positive numbers a and b?

答案是f(x)=-3x.我想知道这答案是如何得到的。

177.When a certain tree was first planted, it was 4 feet tall, and the height of the tree increased by a constant amount each year for the next 6 years.  At the end of the 6th year, the tree was 1/5   taller than it was at the end of the 4th year.  By how many feet did the height of the tree increase each year?
                

答案是2/3,我算了好几次，都得不出是这个答案，请指教。我想要详细过程

答案是2/3,我算了好几次，都得不出是这个答案，请指教。我想要详细过程

63  f(a+b)=-3(a+b)=-3a+(-3b)=f(a)+f(b)

177 设每年增长x   (4+4x)*(1+1/5)=4+6x   解出X=2/3 

第一题是不是4/23

因为48张牌，其中8张代表了股票

前两张不是，则46张牌，其中8章代表了股票

第三张牌是股票的概率就是8/46

那个题应该是选E吧，就是C(8,1)/C(46,1), 其实你可以把之前的两张去除了

这种题最方便的就是那题目里的变量去套用答案里的函数，不需要求解的