Question

expand (x-3a)^-3

See my answer in comments

Answer 1

\[x ^{-3}-3.x ^{-4}.(-3a)+\frac{ -3(-3-1) }{ 2! }x ^{-5}(-3a)^{2}+ \frac{ -3(-3-1)(-3-2) }{ 3! }x ^{-6}(-3a)^{3}\]

Answer 2

\[x ^{-3}+9x ^{-4}a+54x ^{-5}a ^{2}+270x ^{-6}a\]

Answer 3

I get all positive answers, yet i know that every second term ie \[9x ^{-4}a\] and \[270x ^{-6}a ^{3}\] are supposed to be negative.

Answer 4

any suggestions as to why I am an idiot?

Answer 5

i would have used the formula...
\((a-b)^3 = a^3-3a^2b+3ab^2-b^3\)
unless you have typed your question incorrectly...

Answer 6

a=x,b=3a

Answer 7

no its supposed to be to power -3. I have to use binomial theorem

Answer 8

n = -3

Answer 9

blast. i typed it wrong in first post

Answer 10

so, why did you take 2nd term = (-3a)

Answer 11

2nd term wil be 3a
if you are using formula which contain - in middle

Answer 12

2nd term wil be -3a
if you are using formula which contain + in middle

Answer 13

the binomial theorem states \[(x+b)^{v} = x ^{n}+nx^{n-1}b+\frac{ n(n-1) }{ 2! }x^{n-2}b^{2}+\frac{ n(n-1)(n-2) }{ 3! }x^{n-3}b^{3}+...+b^{n}\]

Answer 14

yes, so your 2nd term = -3a
and all signs in between will be +
yet you've taken '-' in between

Answer 15

2nd term = +3x^{-4}(-3a)
okk...so you got it now?

Answer 16

which you wanted..

Answer 17

\[-3.x ^{-5}.(-3a) = 9x ^{-4}a\] am I right?

Answer 18

why '-' in beginning ??

Answer 19

as n = -3

Answer 20

\[(x-3a)^{-3}\]
a=x
b=-3a
n=-3

Answer 21

\[na ^{n-1}b\]

Answer 22

oh wait!
all terms positive is correct!
who said you are incorrect? :P

Answer 23

my text book.

Answer 24

then there is a typo in your textbook
http://www.wolframalpha.com/input/?i=%28x%E2%88%923a%29%5E%28%E2%88%923%29

Answer 25

Thanks man.

Answer 26

welcome ^_^