binomial theorem - help a.s.a.p please

[maths_arghh234]

okay so if this isn't in the right topic .. then pease don't point it out .... i've got binomial theorem past exam questions 2 be given in for 2morrow... and i understand hwo 2 expand nd simplify roughly anyway. it's in the part b's where i get stuck nd i would ask my teacher but it's the way he teaches that gets me stuck in the 1st tbh ... so u can see my problem (hopefully) . ..

okay so 1 of the questions

a..expand (a+b) to the power 4 . (i can do that bit) . hence expand (3x-1/3x) simplifying each term in the expansion. .. okay so i can do them seperatly .. but that must be wrong otherwise they wouldn't put them in the same question would they?...so do i substitute a+b into 3x-1/3x ....???

b.) the coefficient of x squared in the expansion of (I just gt an idea but i'll write it dwn nyway) (1+2x) to the power of n is 40.given that n is a positive interger , find the value of n .. my idea was .. trial and error is it ?? or is there another way ??...

thats 1 of the 6 i've gotto do i hope i get an answer as soon as possible .. maybe a little help on how 2 do this one might help me on the others .. that's wat i hoping anyway

 

[wjm11]

a..expand (a+b) to the power 4 . (i can do that bit) . hence expand (3x-1/3x) simplifying each term in the expansion. .. okay so i can do them seperatly .. but that must be wrong otherwise they wouldn't put them in the same question would they?...so do i substitute a+b into 3x-1/3x ....???

Sure, just do the substitution, as you said. Let a = 3x and let b = -1/3x.

b.) the coefficient of x squared in the expansion of (I just gt an idea but i'll write it dwn nyway) (1+2x) to the power of n is 40.given that n is a positive interger , find the value of n .. my idea was .. trial and error is it ?? or is there another way ??...

No, not trial and error. There really is an easy way to find this. You just need to become familiar with Pascal’s Triangle and how it can be used to expand binomials. Right below the section on Pascal’s Triangle is “shortcut” method for doing the expansion also. Check it out here:

http://www.purplemath.com/modules/binomial.htm

If you study and understand the examples at purplemath, you should have no problem.

 

[maths_arghh234]

hey sorry it's been a late reply (i didn't put the site on favourite and lost it ) thank for your help ... and i found the site really useful .. i'm still not great with it but better ...hopefully