The Student Room Group

C4 Arghhhh!!!

Can anyone help me with this TORTUREEEE!!

1) (1+kx)(1+4x)^½ - in the expansion of this, the coefficient of x is 7. Find the value of the constant k and hencee the coefficient of x².

2) Expand - (1+3x)^-1 |x|<1/3
i get hw to expand but thennnn..
Hence show that for small x..
(1+x)/(1+3x) &#8776; 1-2x+6x²-18x³
THEN..
take a suitable value for x which should be stated..use the series expansion in part b to find an approximate value for 101/103.

:biggrin: :biggrin: :biggrin: cheers
Reply 1
Can anyone help me with this TORTUREEEE!!

1) (1+kx)(1+4x)^½ - in the expansion of this, the coefficient of x is 7. Find the value of the constant k and hencee the coefficient of x².

2) Expand - (1+3x)^-1 |x|<1/3
i get hw to expand but thennnn..
Hence show that for small x..
(1+x)/(1+3x) &#8776; 1-2x+6x²-18x³
THEN..
take a suitable value for x which should be stated..use the series expansion in part b to find an approximate value for 101/103.

:biggrin: :biggrin: :biggrin: cheers
Reply 2
in part b, do the whole thing then substitute for x = 0.01
Reply 3
in part b, do the whole thing then substitute for x = 0.01
Reply 4
still dont get it..anyone??
Reply 5
still dont get it..anyone??
Reply 6
Fizzle
Can anyone help me with this TORTUREEEE!!

1) (1+kx)(1+4x)^½ - in the expansion of this, the coefficient of x is 7. Find the value of the constant k and hencee the coefficient of x².

2) Expand - (1+3x)^-1 |x|<1/3
i get hw to expand but thennnn..
Hence show that for small x..
(1+x)/(1+3x) &#8776; 1-2x+6x²-18x³
THEN..
take a suitable value for x which should be stated..use the series expansion in part b to find an approximate value for 101/103.

:biggrin: :biggrin: :biggrin: cheers


(1+kx)(1+4x)^½ = (1+kx)(1 + (0.5)4x + (0.5*-0.5/2)(4x)^2...)
equation x terms, 7 = 2 + k, so k = 5

so x^2 terms = 4*2 + (0.5*-0.5/2)4^2 = 8 - 2 = 6

probably wrong.
Reply 7
Fizzle
Can anyone help me with this TORTUREEEE!!

1) (1+kx)(1+4x)^½ - in the expansion of this, the coefficient of x is 7. Find the value of the constant k and hencee the coefficient of x².

2) Expand - (1+3x)^-1 |x|<1/3
i get hw to expand but thennnn..
Hence show that for small x..
(1+x)/(1+3x) &#8776; 1-2x+6x²-18x³
THEN..
take a suitable value for x which should be stated..use the series expansion in part b to find an approximate value for 101/103.

:biggrin: :biggrin: :biggrin: cheers


(1+kx)(1+4x)^½ = (1+kx)(1 + (0.5)4x + (0.5*-0.5/2)(4x)^2...)
equation x terms, 7 = 2 + k, so k = 5

so x^2 terms = 4*2 + (0.5*-0.5/2)4^2 = 8 - 2 = 6

probably wrong.
Reply 8
thats GENIOUS!!!!!!!!!!!!!!!!!!! :biggrin: thanks a lot darl! :smile:
Reply 9
thats GENIOUS!!!!!!!!!!!!!!!!!!! :biggrin: thanks a lot darl! :smile:
Reply 10
the second one still has me baffled tho.. i bet its dead easy but im jus bein a dummy as usual..!
Reply 11
the second one still has me baffled tho.. i bet its dead easy but im jus bein a dummy as usual..!
1. (1+kx)(1+4x)1/2

=(1+kx)(1+2x+½(½)(-½)(4x)²+...)
=(1+kx)(1+2x-2x²+...)
=1+(2+k)x+(2k-2)x²+...

2+k=7 so k=5

so coefficient of = 2(5)-2=8

2. (1+3x)-1 = 1-3x+9x²-27x³+...

=&#8721;(-3x)n from n=0 to n=infinity

(1+x)/(1+3x)=(1+x)(1+3x)-1

=(1+x)(1-3x+9x²-27x³+...)
=1-2x+6x²-18x³+...

let (1+x)/(1+3x)=101/103=1.01/1.03

so x=0.01=1/100

then 101/103&#8776;1-2/100+6/10000-18/1000000

then calculate what this is.
1. (1+kx)(1+4x)1/2

=(1+kx)(1+2x+½(½)(-½)(4x)²+...)
=(1+kx)(1+2x-2x²+...)
=1+(2+k)x+(2k-2)x²+...

2+k=7 so k=5

so coefficient of = 2(5)-2=8

2. (1+3x)-1 = 1-3x+9x²-27x³+...

=&#8721;(-3x)n from n=0 to n=infinity

(1+x)/(1+3x)=(1+x)(1+3x)-1

=(1+x)(1-3x+9x²-27x³+...)
=1-2x+6x²-18x³+...

let (1+x)/(1+3x)=101/103=1.01/1.03

so x=0.01=1/100

then 101/103&#8776;1-2/100+6/10000-18/1000000

then calculate what this is.
Reply 14
brilliant! thanks so much guys :biggrin:
Reply 15
brilliant! thanks so much guys :biggrin: