Algebra Strategy Guide - Chapter 6 - Pg 85. Question #8

[deem432]

Question #8:

If a(sub n) = ( a(sub n-1) x a(sub n-2) ) / 2
and a(sub 5) = -6 and a(sub 6) = -18, what is the value of a(sub 3)

I read the explanation in the book and am struggling to figure out why my math below does not work:

-18 = ( -6 x a(sub 4) ) / 2
-18 = -3 x a(sub 4)
-18 + 3 = a(sub 4)
-15 = a(sub 4)

a(sub 4) is 6 and not -15. I think I shouldn't have divided -6 by 2 and instead should have multiplied -18 by 2. I get the correct answer when I do the latter. Can someone please explain to me why I shouldn't divide -6 by 2? I don't want to make a similiar mistake like this in the future.

[RonPurewal]

your error is the red part:

-18 = ( -6 x a(sub 4) ) / 2
-18 = -3 x a(sub 4)
-18 + 3 = a(sub 4)
-15 = a(sub 4)

if something is multiplied by -3, you can't undo that with addition. you have to divide by -3.

[RonPurewal]

... and there's nothing wrong with your first step (in which you simplified -6/2 = -3).