Question

what is the fifth term of the expansion of (a-3b)^5

Answer 1

choices are
a)405ab^4
b)405ab^5
c)81ab^4
d/-405ab^4

Answer 2

Are you using binomial theorem?

Answer 3

yes

Answer 4

Ok, cool. So, we need to identify the x, y, and r in the formula.

Answer 5

?

Answer 6

Does your binomial theorem look like this: http://www.mathwarehouse.com/algebra/polynomial/binomial-theorem.php Look about halfway down the page

Answer 7

similar

Answer 8

Does yours use different letters?

Answer 9

so far i got 5C4a^0b^4

Answer 10

yes

Answer 11

Hm, I see how you got b^4, but I don't get a^0

Answer 12

since its the fifth term wouldn't the exponent be 0

Answer 13

or 1. i dont know

Answer 14

I get the fifth term to have x^(r-4) which would make a^(5-4)

Answer 15

Yep, which is a^1

Answer 16

so what would be the answer

Answer 17

Ok, I see how you would get what you put before, but I would make another slight adjustment.
5C4a^1(3b)^4

Answer 18

Does that form make sense?

Answer 19

ok

Answer 20

do u know what 5c4 is

Answer 21

Yes, that is a combination. Most calculators have a combination button.

Answer 22

Oh, and I made a slight error. It should be
5C4a^1(-3b)^4

Answer 23

Do you see a button on your calculator that looks like nCr?

Answer 24

If not, you can use this formula: