Find an explicit rule for the nth term of a geometric sequence where the second and fifth terms are -12 and 768, respectively.

an = 3 • (-4)n + 1
an = 3 • 4n - 1
an = 3 • (-4)n - 1
an = 3 • 4n

Question

Find an explicit rule for the nth term of a geometric sequence where the second and fifth terms are -12 and 768, respectively.

 an = 3 • (-4)n + 1
 an = 3 • 4n - 1
 an = 3 • (-4)n - 1
 an = 3 • 4n

anonymous · Answer

As we know Un=Up.r^n-p right?

misssunshinexxoxo · Answer

Which do you believe it is?

anonymous · Answer

honestly i have no clue how to do this

misssunshinexxoxo · Answer

T(n)=T(1)r^(n-1) 
=> 
T(1)r=36--------(1) 
T(1)r^4=2304--------(2) 
(2)/(1)=> 
r^3=2304/36=> 
r=4 
From (1), get 
T(1)*4=36=> 
T(1)=9 
=> 
the general term  
T(n)=9*4^(n-1)

anonymous · Answer

Hey are you sure you have written the choices correctly ?

anonymous · Answer

can i just plug them in and see which one gives me the 2nd as -12 and the 5th as 768

anonymous · Answer

yea

anonymous · Answer

Well yea thats the only way to solve but none works

anonymous · Answer

@misssunshinexxoxo which choice would it be?

misssunshinexxoxo · Answer

My best answer would be  an = 3 • (-4)n - 1 should be good for this

anonymous · Answer

Wait r=-4 and not 4