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Core 4 binomial expansion Q

image.jpgFrom AQA MPC4 Jun 13

Question is attached; I have no idea how to do the last part of the Q

Any guidance is very much appreciated, thanks
What happens when x = 1/6 ?
Reply 2
Avatar for B_9710
B_9710
18
We want to use our expansion to approximate the cube root. So we plug x=1/6 into the expansion as this makes the denominator 28.
However we need 2 lots of this so we multiply the expansion in bi) by 2. Then just use a calculator to give the answer to 6dp.
Just say if you need any more help cos I don't think I have explained this too well.
Original post by EricPiphany
What happens when x = 1/6 ?


Original post by B_9710
We want to use our expansion to approximate the cube root. So we plug x=1/6 into the expansion as this makes the denominator 28.
However we need 2 lots of this so we multiply the expansion in bi) by 2. Then just use a calculator to give the answer to 6dp.
Just say if you need any more help cos I don't think I have explained this too well.


i dont understand why the jump to x=1/6

?
Original post by hi-zen-berg
i dont understand why the jump to x=1/6

?


Becuase when subbed into (27+6x)^{-1/3} it gives us yummy yummy which we need
Let 27+6x=28
Reply 6
Avatar for B_9710
B_9710
18
You want the denominator to be 28 so that you have cube root of 28 on denominator. So 27+6x=28
So x=1/6 so you sub that value into your expansion.
Original post by B_9710
You want the denominator to be 28 so that you have cube root of 28 on denominator. So 27+6x=28
So x=1/6 so you sub that value into your expansion.


Thanks
Original post by EricPiphany
Let 27+6x=28


Original post by B_9710
You want the denominator to be 28 so that you have cube root of 28 on denominator. So 27+6x=28
So x=1/6 so you sub that value into your expansion.


ahhhh i see

thanks guys, big help

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