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# need help on these two questions! binominal exapnsion. please! watch

1. 1.) find the coefficient of x^4 in the expansion of (1+3x)^7
2.) complete the first 3 terms in the bionominal expansion of (1=6x)^9
use thee terms to find an approximate value for (1.06)^9 correct to 3dp

3. my working i like tooo long and complicated i used the rule:
1+nx+n(n-1)x^2+n(n-1)(n-2)x^3
but still semm to get it wrong and confused
4. i got a answer of 840 x^4 for the first 1 :/
5. (Original post by Humii)
my working i like tooo long and complicated i used the rule:
1+nx+n(n-1)x^2+n(n-1)(n-2)x^3
but still semm to get it wrong and confused
You don't appear to be dividing by factorials.

1+nx+n(n-1)x^2/2! +n(n-1)(n-2)x^3/3!
6. yh, i did but didnt knw hw to write it on here
7. first one (sorry havent got my calculator so cant work out) is 7C4 x 1^3 x 3^4 x (x)^4

second one is 1 + 9 x 1 x 6x + 9C2 x 1 x 6^2 x (x)^2

for the second part of two u make (1+6x) = 1.06 so x = 0.01, and then u substitue x into the equation above, so 1 + 9 x 1 x 6(0.01) etc... and this should give u an answer to (3.d.p)

hope this helps

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Updated: March 20, 2011
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