Prelim 2016 Maths Help Thread (1 Viewer)

[parad0xica]

Also need help with this question:
find the value of x when
cos2θ<=1/√2, when 0<=θ<=2π

At least have a try... it's best to DIY then seek help during emergencies

 

[Nailgun]

Also need help with this question:
find the value of x when
cos2θ<=1/√2, when 0<=θ<=2π

idk if im doing something wrong lelele
is the answer 5pi/8 and pi/8 ?

 

[Green Yoda]

idk if im doing something wrong lelele
is the answer 5pi/8 and pi/8 ?

It's an inequality..below y=1/sqrt(2)..

 

[InteGrand]

Also need help with this question:
find the value of x when
cos2θ<=1/√2, when 0<=θ<=2π

Try sketching the graph of the function and figure it out from that.

 

[Nailgun]

It's an inequality..below y=1/sqrt(2)..

oh whoops didnt see the <

rip

i legit do dumbass things everytime I try and do post maths here lel
i feel stupid now lelel

 

[Green Yoda]

At least have a try... it's best to DIY then seek help during emergencies

I have obviously tried alot lol..I can't seem to figure out a way of doing this..other than drawing a cos2theta graph with the units being in radians..which I can't currently do as I'm not that good at radians atm..

 

[InteGrand]

I have obviously tried alot lol..I can't seem to figure out a way of doing this..other than drawing a cos2theta graph with the units being in radians..which I can't currently do as I'm not that good at radians atm..

If you can sketch the graph of y = f(x) = cos(theta), then the graph of y = g(x) = cos(2*theta) is just obtained by contracting the graph of y = f(x) horizontally by a factor of 2.

 

[leehuan]

A challenge question for the 3u kids:
if secθ-tanθ= 3/5
show sinθ=8/17

Cbf typing theta atm

A tedious way to approach this would be to say that from secx-3/5=tanx, assuming cosx can't equal to 0 we have 1-3/5sinx=cosx, and then square the equation. However, cos^2x can be replaced using the Pythagorean identity cos^2x=1-sin^2x.
Then you are effectively solving a quadratic in terms of sinx.

But because I haven't tried the method myself, I'm a bit cautious as to how to discard the wrong case.

 

[parad0xica]

I have obviously tried alot lol..I can't seem to figure out a way of doing this..other than drawing a cos2theta graph with the units being in radians..which I can't currently do as I'm not that good at radians atm..

How is it obvious that you tried a lot?

You posted none of your progress and have not pinpointed precisely where the problem is.

 

[Green Yoda]

Cbf typing theta atm

A tedious way to approach this would be to say that from secx-3/5=tanx, assuming cosx can't equal to 0 we have 1-3/5sinx=cosx, and then square the equation. However, cos^2x can be replaced using the Pythagorean identity cos^2x=1-sin^2x.
Then you are effectively solving a quadratic in terms of sinx.

But because I haven't tried the method myself, I'm a bit cautious as to how to discard the wrong case.

there is a simpler way..but ill let the some prelim kids do it

 

[Green Yoda]

How is it obvious that you tried a lot?

You posted none of your progress and have not pinpointed precisely where the problem is.

oh..bos is like my last option if i dont get it lol and probably my only
i tried a lot..but I couldn't do it

 

[Nailgun]

At least have a try... it's best to DIY then seek help during emergencies

I think generally speaking people only post problems for help when they've already tried and failed in doing them

 

[Drongoski]

 

[Green Yoda]

Hmm interesting method..
I did it via t formula and found the value of t and then subbed that into the sin t formula and proved it like that

 

[leehuan]

Hmm interesting method..
I did it via t formula and found the value of t and then subbed that into the sin t formula and proved it like that

That was basically the exact same method I suggested.

Use of t-formula becomes a bit awkward because you have to make 2t/(1+t^2) the subject

 

[parad0xica]

I think generally speaking people only post problems for help when they've already tried and failed in doing them

Why don't they post working out?

 

[Nailgun]

Why don't they post working out?

generally because laziness lol
that being said, while if you do post up working out then someone can correct whatever mistake you did
but you can also see where you went wrong by looking at someone else's working (which they likely would have written up anyways once they pointed out what you did wrong)
so its kinda more efficient in the end

tbh it might be more beneficial understanding where you went wrong yourself by examining someone else's solution rather than someone telling you

 

[Green Yoda]

still dont know how to do that question lol
help plox

 

[InteGrand]

still dont know how to do that question lol
help plox

Have you tried sketching the graph of the function? You should probably master that first, as it is helpful for these types of Q's, and other Q's.

 

[leehuan]

Because you probably don't have the skill to visualise it just yet, seriously draw out a graph.