Author Topic: Maths P1 doubt on markscheme  (Read 1954 times)

Offline cashem'up

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Maths P1 doubt on markscheme
« on: May 09, 2010, 05:01:55 am »
question from s06 p1 maths......... i have a doubt the ms gives only two values but i get 4.........

sin 2x + 3cos2x = 0 between 0 and 180 solve....

initially i divided by cos2x to get tan2x=3 and found two values 54.2 and 144.2
but then i did

cos2x(tan2x+3)=0 by taking cos 2x in common then i also get values of 45 and 135 but its not there in the MS........have i done any mistake.........help.

Offline Meticulous

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Re: Maths P1 doubt on markscheme
« Reply #1 on: May 09, 2010, 06:29:26 am »
Divide both sides by cos2x:
Tan2x+3=0
Tan2x=-3
2tanx/1-tan2x=-3
2tanx=-3+3tan2x
3tan2x-2tanx-3=0
Tanx= 1.387, or tanx= -0.720
Therefore, x= 54.2, -35.75
Now use the tan graph or the ASTC rule to find the other angles possible:
X= 54.2, 234.2 -35.75, 144.5

Therefore, x= 54.2, 144.5

nid404

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Re: Maths P1 doubt on markscheme
« Reply #2 on: May 09, 2010, 06:37:34 am »
sin2x+3cos2x=0

divide by cos2x u get

tan2x+ 3=0

tan2x=-3
2x= tan-1 (-3)
2x=-71.6
x=-35.5
the range is 0-180

period of a tan curve is pi...in this case pi/2 (since it's tan 2x)

x=-35.5+180 =144.5
x=-35.5+90=54.5

Offline Meticulous

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Re: Maths P1 doubt on markscheme
« Reply #3 on: May 09, 2010, 06:45:27 am »
You use the periodicity method. Quite helpful I must say.

nid404

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Re: Maths P1 doubt on markscheme
« Reply #4 on: May 09, 2010, 06:47:15 am »
yup. Faster  ;)

Offline cashem'up

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Re: Maths P1 doubt on markscheme
« Reply #5 on: May 09, 2010, 07:57:31 am »
yup. Faster  ;)

hey thanks guy nid404 and gladiator......... i did the same as wat nid did ...... by the periodicity method however in the 2nd method i attempted i am getting 4 answers where  took cos2x is taken common... how is it wrong......

nid404

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Re: Maths P1 doubt on markscheme
« Reply #6 on: May 09, 2010, 07:59:51 am »
can you show how you derived your answer?

I can then help rectify

Offline Meticulous

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Re: Maths P1 doubt on markscheme
« Reply #7 on: May 09, 2010, 08:25:08 am »
Use periodicity when you're PERFECT at using it. Other wise you'll lose track.

Offline cashem'up

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Re: Maths P1 doubt on markscheme
« Reply #8 on: May 09, 2010, 09:28:33 am »
can you show how you derived your answer?

I can then help rectify

see i have
sin 2x + 3cos2x = 0
now if i take cos 2x common then
cos2x(sin2x/cos2x +3)
so cos2x(tan2x+3)=0
now one ans is tan2x= -3 and as u did by the periodicity method above get 2 answers
however another would also be cos2x=0 and so u get 2 more answers 45 and 135...................... would that be correct i dont find nethin wrong in it except the fact that its not in the MS

nid404

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Re: Maths P1 doubt on markscheme
« Reply #9 on: May 09, 2010, 09:53:36 am »
wait...how on earth do you get cos2x=0 ?

Offline Meticulous

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Re: Maths P1 doubt on markscheme
« Reply #10 on: May 09, 2010, 10:21:31 am »
He factorised the equation. So now either cos2x=0, or tan2x+3=0

nid404

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Re: Maths P1 doubt on markscheme
« Reply #11 on: May 09, 2010, 10:34:53 am »
aaah.....ok..

Should give you the same answer. I'll try

Offline cashem'up

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Re: Maths P1 doubt on markscheme
« Reply #12 on: May 09, 2010, 12:09:42 pm »
guyzz....ne solution any idea...........???  ???

Offline Meticulous

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Re: Maths P1 doubt on markscheme
« Reply #13 on: May 09, 2010, 12:22:51 pm »
Use my method. Works all the time. Takes time, but it's structured and pretty good.

nid404

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Re: Maths P1 doubt on markscheme
« Reply #14 on: May 09, 2010, 12:54:31 pm »
ohk....now i realize what's wrong in ur case.

you cannot take cos2x common...

they are 2 separate terms.