# Using double identity, show… = tan²x

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#1
It seems that to be trig identities is definitely my weakest link in maths. I keep doubting myself

Show 1-cos 2x ÷ 1+ cos 2x=tan ² x

The only way that I can link cos with tan is to use the tan x=Sin x÷ Cos x

I think I have the right idea but my result is negative 2tan²x

See attached:

Upon writing this, I believe I have failed to expand the bracket correctly, so it would be 1-1+2sin²x÷ 1+ 2cos²x.

I can’t use cos 2x=cos ²x-sin²x as this would be tan ²x=0
Last edited by KingRich; 1 month ago
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1 month ago
#2
(Original post by KingRich)
It seems that to be trig identities is definitely my weakest link in maths. I keep doubting myself

Show 1-cos 2x ÷ 1+ cos 2x=tan ² x

The only way that I can link cos with tan is to use the tan x=Sin x÷ Cos x

I think I have the right idea but my result is negative tan²x

See attached:
There's double angles on the LHS but a single angle on the RHS. This instructs you to use the double angle identities and simplify from there.

If you have done this and are getting -tan^2 then that just means you're making a sign error somewhere. Maybe you're not substituting in the correct signs when applying double angle identities.

EDIT: looking at your attachment, you didn't expand the numerator correctly. Also, don't lay out your work like this when writing the answer in the exam. It's better to lay it out as:

(LHS) = .... = .... = .... = (RHS)

whereas you are laying it out as

(LHS) = (RHS)
.... = (RHS)
.... = (RHS)
.... = (RHS)
(RHS) = (RHS)
Last edited by RDKGames; 1 month ago
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#3
(Original post by RDKGames)
There's double angles on the LHS but a single angle on the RHS. This instructs you to use the double angle identities and simplify from there.

If you have done this and are getting -tan^2 then that just means you're making a sign error somewhere. Maybe you're not substituting in the correct signs when applying double angle identities.

EDIT: looking at your attachment, you didn't expand the numerator correctly. Also, don't lay out your work like this when writing the answer in the exam. It's better to lay it out as:

(LHS) = .... = .... = .... = (RHS)

whereas you are laying it out as

(LHS) = (RHS)
.... = (RHS)
.... = (RHS)
.... = (RHS)
(RHS) = (RHS)
Okay. I see what you mean.

mmm, you mean the part when I’ve drawn an arrow. I had corrected this in my original comment.

But upon inspection, I believe I am correct. As 2 would cancel out and hence leave tan²x
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1 month ago
#4
(Original post by KingRich)
Okay. I see what you mean.

mmm, you mean the part when I’ve drawn an arrow. I had corrected this in my original comment.

But upon inspection, I believe I am correct. As 2 would cancel out and hence leave tan²x
Be careful with the brackets, -(1-2sin^2x) is -1+2sin^2x
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#5
(Original post by Skiwi)
Be careful with the brackets, -(1-2sin^2x) is -1+2sin^2x

But thank you for confirming
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1 month ago
#6
(Original post by KingRich)
It seems that to be trig identities is definitely my weakest link in maths. I keep doubting myself

Show 1-cos 2x ÷ 1+ cos 2x=tan ² x

The only way that I can link cos with tan is to use the tan x=Sin x÷ Cos x

I think I have the right idea but my result is negative 2tan²x

See attached:
You MUST NOT put the expressions equal to each other - that is a huge error.

You need to cinnsider the LHS and RHS separately.
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#7
(Original post by Muttley79)
You MUST NOT put the expressions equal to each other - that is a huge error.

You need to cinnsider the LHS and RHS separately.

to confirm, is this layout correct?

Or.

Last edited by KingRich; 1 month ago
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1 month ago
#8
here is how you can set out a typical trig proof:

keep the left side separate from the right side.

work on both sides until you get a match
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1 month ago
#9
(Original post by KingRich)

to confirm, is this layout correct?
No - split the two sides.

LHS triple equals .... [is equivalent to, not just equals]

and show you get the RHS

or vice versa.

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#10
(Original post by Muttley79)
No - split the two sides.

LHS triple equals .... [is equivalent to, not just equals]

and show you get the RHS

or vice versa.

I don’t have a teacher. I’m home studying the whole a level myself, which is why I ask a lot of questions on here for support
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#11
(Original post by the bear)
here is how you can set out a typical trig proof:

keep the left side separate from the right side.

work on both sides until you get a match
So, you’re saying I should draw a line down the middle to show them as separate? I thought it looked clear there’s a LHS and RHS
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1 month ago
#12
(Original post by KingRich)
So, you’re saying I should draw a line down the middle to show them as separate? I thought it looked clear there’s a LHS and RHS
i am not saying anything. you can do what you like.
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#13
(Original post by the bear)
i am not saying anything. you can do what you like.
Well, suggesting. Lol
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#14
(Original post by Muttley79)
No - split the two sides.

LHS triple equals .... [is equivalent to, not just equals]

and show you get the RHS

or vice versa.

As bear suggested.

would I have to something like this when dealing with identities? Clearly labelled
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1 month ago
#15
(Original post by KingRich)
As bear suggested.

would I have to something like this when dealing with identities? Clearly labelled
Don't write tan^2 at the top at all write LHS triple equals etc

You just work on one side not both.

State which rule you are using to move from one line to the next

Finish when you get the RHS exactly.
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1 month ago
#16
(Original post by KingRich)

But thank you for confirming
Didn't see the response, apologies
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#17
(Original post by Muttley79)
Don't write tan^2 at the top at all write LHS triple equals etc

You just work on one side not both.

State which rule you are using to move from one line to the next

Finish when you get the RHS exactly.
Mmm, okay. I’ll take the advice and put it to work and please let me know next time if I’m still making the error. Thanks
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1 month ago
#18
Without wanting to prolong this,
https://www.tuitionmath.com/single-p...ometry-proving
has a few decent tips about how to go about common trig identity questions (and how to spot them) as well as a reasonable way to write them up. Pretty much as hinted at as above, and there should be similar examples in your textbook.
Last edited by mqb2766; 1 month ago
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#19
(Original post by mqb2766)
Without wanting to prolong this,
https://www.tuitionmath.com/single-p...ometry-proving
has a few decent tips about how to go about common trig identity questions (and how to spot them) as well as a reasonable way to write them up. Pretty much as hinted at as above, and there should be similar examples in your textbook.
Lol, I think I just acknowledged my error.

I believe this to make better sense

Or,

Last edited by KingRich; 1 month ago
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1 month ago
#20
(Original post by KingRich)
Lol, I think I just acknowledged my error.

I believe this to make better sense
Thats certainly an improvement. A few minor things,
* Dont overuse brackets (as in the previous posts). tan^2(x) is much better than (tan^2 x)
* Put a small amount of explanation in when youre using a double angle identity or whatever. Should the marker read down the left column first, then start at the top of the right column or .... of your answer?
* If you start at the LHS of the desired identity, say something like that at the start.
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