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need help!

alright, the first q is on integration technics.

1) f(x) = 16 + 2x + 15x^2 / (1+x^2)(2-x) = A+BX / (1+x^2) + C/ (2-x)

A=0 B=1 and C=16 to give

f(x) = 16 + 2x + 15x^2 / (1+x^2)(2-x) = X / (1+x^2) + 16/ (2-x)

next i had to integrate it between 1 and 0 and got 33/2 ln2

the next part is where i need the help. i dont really understand what its asking for:s-smilie:.

Express f(x) as a sum of powers of x up to and including x^4. determine the range of values of x for which this expansion of f(x) is valid

the next q is about max and min values

2) i got parametric equations of:

x=2cos(theta) + sin (theta)
y= cos (theta) + 2sin (theta)

for 0 </=x<2pi

i had to express x in the form R cos (theta-alpha): ans= root5 cos (theta - 0.4636)

next i had to find the graient ar point where theta= pi/2 ans=0.5

then i had to show x^2 + y^2 =5 + 4sin2 theta which i did.

i had a go at this but couldnt do it.

Find the greatest and least distances of a point on the curve from the origin.

i had a go at differentiating it again...but that didnt work, but i just noticed i used the values 0 and 2pi. but i failed on that, i couldnt thonk of anything else.:mad:
Reply 1
1)use maclaurin series
differentiate f(x) and then f'(x) and f''(x) etc
find the values for these when x=0
f(x)=f(0)+ f'(0)x + (f''(0)x^2)/2! + ....

2)x=rcostheta, y= rsintheta

r^2= 5+4sin 2theta
r= (5+4sin 2theta)^0.5
dr/dtheta= 4cos 2theta/(5+4sin2theta)^0.5

dr/dtheta= 0 for max and min r
Sorry OP, this is off-topic, but bobo - you're in Year 11 and you're going to do STEP III?
Reply 3
cos 2theta=0

2theta= -(pi/2), pi/2
thetA= -(pi/4), pi/4

r= (5+4)^0.5= 3 (max r)
r=(5-4)^0.5=1 (min r)
Reply 4
Suc-Cesc!!!
Sorry OP, this is off-topic, but bobo - you're in Year 11 and you're going to do STEP III?


yes, i'm ready to do them so theres no point in waiting till a 2 years time when the topics covered won't be fresh in my mind
*bobo*
yes, i'm ready to do them so theres no point in waiting till a 2 years time when the topics covered won't be fresh in my mind


fookin hell, that's amazing. Good luck :yy:
Reply 6
IsOLaTiOnIsT
1) f(x) = 16 + 2x + 15x^2 / (1+x^2)(2-x) = A+BX / (1+x^2) + C/ (2-x)

next i had to integrate it between 1 and 0 and got 33/2 ln2
f(x) = (16+2x+15x^2)/(1+x^2)(2-x)
......= (A+Bx)/(1+x^2) + C/(2-x)
(16+2x+15x^2) = (A+Bx)(2-x) + C(1+x^2)
(16+2x+15x^2) = (2A+2Bx-Ax-Bx^2+C+Cx^2), so 2A+C = 16, 2B-A = 2, C-B = 15

B = 1, A = 0, C = 16

f(x) = x/(1+x^2) + 16/(2-x)

INT f(x).dx [from 0 to 1]
= INT x/(1+x^2) + 16/(2-x).dx [from 0 to 1]
= (1/2) INT 2x/(1+x^2).dx - 16 INT -1/(2-x).dx [from 0 to 1]
= [(1/2)ln(1+x^2) - 16ln(2-x)] [from 0 to 1]
= (1/2)ln(1+1^2) - 16ln(2-1) - (1/2)ln1 + 16ln2
= (1/2)ln(2) + 16ln2
= (33/2)ln(2)
Reply 7
Suc-Cesc!!!
fookin hell, that's amazing. Good luck :yy:


thanx :smile:
Reply 8
*bobo*
1)use maclaurin series
differentiate f(x) and then f'(x) and f''(x) etc
find the values for these when x=0
f(x)=f(0)+ f'(0)x + (f''(0)x^2)/2! + ....

2)x=rcostheta, y= rsintheta

r^2= 5+4sin 2theta
r= (5+4sin 2theta)^0.5
dr/dtheta= 4cos 2theta/(5+4sin2theta)^0.5

dr/dtheta= 0 for max and min r
I would recommend using the binomial for 1)
Reply 9
yep that would be deffinately less time consuming.
Reply 10
year 11 doing STEPS :O....i wish i had a brighter mind :frown:...well done on u, hope u do well :wink: oh thanx for the help to who ever contributed :smile:
Reply 11
*bobo*


2)x=rcostheta, y= rsintheta

r^2= 5+4sin 2theta
r= (5+4sin 2theta)^0.5
dr/dtheta= 4cos 2theta/(5+4sin2theta)^0.5

dr/dtheta= 0 for max and min r


shout at me if im wrong, but if ur differentiating, isn't the power ment to decrease by one and times it by new power :s-smilie:
Reply 12
its been decreased by 1 as 4cos 2theta/(5+4sin2theta)^0.5= 4cos2theta(5+4sin2theta)^(-0.5)
when differentiating you times it by the initial power and the function to that power differntiated(i.e. in this case times by 0.5 x 2 x 4cos2theta= 4cos2theta)
Reply 13
.......i feel so stupid :frown: thanx again
Reply 14
just reading what u suggested for q1) ive never come across ur method, im guessing binomial is the other option, right? :s-smilie:
Reply 15
thats ok. yes, binomial is probably the best method in this case. the maclaurin series is introduced on fp2.
IsOLaTiOnIsT
just reading what u suggested for q1) ive never come across ur method, im guessing binomial is the other option, right? :s-smilie:
Yes, and in fact I would say that is the expected method here.

(In fact, if you do it by Maclaurin expansion, I don't see how you are supposed to give the range for which the expansion is valid, at least within the confines of the A-level syllabus).

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