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Maths & Physics Help

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blah4444:
can u pls explain qn no.11 in the may/june 2008 p2?
thanks

astarmathsandphysics:
use the cosine rule a^2=b^2+c^2-2bc*cosA
21^2=x^2+92x)^2-x*2x*cos120
441=x^2+4x^2-4x^2*-1/2
441=5x^2+4x^2
441=9x^2
so x=sqrt(441/9)=7

igvictim:
hey, thank you for the formulae sheet, it helped  :D

cheers

qwerty:
hey..i have a doubt on factorising stuff

how do i know what method to use?

for example:

6ab - 9bx - 21by

i would easily transform into:

3b (2a - 3x - 7y)

but, for example:

k² + 9k + 8

what should be done?thanks!

astarmathsandphysics:
First is correctt. In second look for 2 numbers that add to give 9 and times to give 8.
The answer is 1 and 8 so it factorises as (x+1)(x+8)

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