GWD22 Q11求解

 求解兩道GWD题,请数學高手帮忙解答,谢谢.

GWD22 Q11

In a stack of cards, 9 cards are blue and the rest are red. If 2 cards are to be chosen at random from the stack without replacement,

the probability that the cards chosen will both be blue is 6/11.What is the number of cards in the stack?

A.10  B.11  C.12  D.15  E.18

PP19

The sequence a₁.a₂....a_n..  is such that a_n=4a_n-1-3  for all intergers n>1. If a₃=x,  then a₁=?

A.4X-3   B.16X-15   C.(X+3)/4    D.(X+3)/16    E.(X+15)/16

1---C;x分之9乘以x-1分之8等于11分之六

2---E； 按照公式往里代，a3=*a2,a2=*a1 => a3=**a1=x