gwd math 25-17

Q17

If P(r)=8r/(1-r), for what value of r does P(r)=1/2(P(3))?

A.      6

B.      3

C.      0

D.     –3

E.      –6

这一题我怎麽算也是-6呀？怎麽会是-3?

since P(r) = 8r/(1-r) = (1/2)*(P(3)) = (1/2) ((8*3)/(1-3))

so: 8r/(1-r) = (1/2) * (24/-2)

so  8r/(1-r) =-6

so  8r = -6 * (1-r)

so 8r = 6r -6

so r = -3