Binomial Expansion Core 2

11

Please can someone help me with thhis question...
2(a) Find the first 4 terms, in ascending powers of x, the binomial expansion of (1-2x)^5 - Which i understand
It's part b that i don't...
b) if x is small, so that x^2 and high powers can be ignored, show that (1+x)(1-2x)^5 = 1-9x
Thanks

Reply 1

ste0731

12

stanmoor

Please can someone help me with thhis question...
2(a) Find the first 4 terms, in ascending powers of x, the binomial expansion of (1-2x)^5 - Which i understand
It's part b that i don't...
b) if x is small, so that x^2 and high powers can be ignored, show that (1+x)(1-2x)^5 = 1-9x
Thanks

Write out (1-2x)^5 (just use the terms involving 1 and -10x (I think it is)

Then multiply that by (1+x)

so you have (1+x)(1-10x)=1-10x+x-10x^2

=1-9x-10x^2
however x^2 can be ignored for small x
Therefore = 1-9x as required

Reply 2

TropLouis

I think you just need to multiply the first 2 terms of the expansion by (1+x) and you'll get the answer :smile:

Reply 3

stanmoor

OP

11

ah i understand! thankyou!

Binomial Expansion Core 2

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