Post a New Question

Math

posted by .

Find a1 in a geometric series for which Sn=189,r=1/2,and an=3.

Sn=a1(1-r^n)/1-r
189=a1(1-1/2^3)/1-1/2
189=a1(-.5)/.5
189/-.25
a1=-756

  • Math -

    YOur third line is wrong

    should be:

    189 = a(1 - 1/8)/(1 - 1/2)
    189 = a(7/8)/(1/2)
    189 = a(7/4)
    756 = 7a
    108 = a

    (check 108+54+27 = 189 !)

Answer This Question

First Name:
School Subject:
Answer:

Related Questions

More Related Questions

Post a New Question