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C3 trig. question



Not sure how to finish this question off.

Basically at the moment what I have tried is writing out sin(x-A) and cos(x+A) in their full form using the compound angle formulae; then trying to cancel out the A's and make tanx out of sinx/cosx, but I can't seem to cancel it down that far.

any help is greatly appreciated :confused:
Reply 1
Write out your working in full, the method seems sound
Reply 2
sin(X-A)/cos(X+A)=1
sinXcosA-cosXsinA/cosXcosA-sinXsinA=1
(sinXcosA-cosXsinA/cosAcosX)/(cosXcosA-sinXsinA/cosAcosX)=1
(sinX/cosX-sinA/cosA)/1-tanXtanA=1
tanX-tanA/1-tanXtanA=1
tanX-tanA=1-tanXtanA

This is what I've got so far but i have no idea where to go from here
Reply 3
muster666
sin(X-A)/cos(X+A)=1



how?

if u rearrange the equation u get : sin(X-A) - cos(X+A) = 0
Reply 4
I just thought maybe I should write:
sin(X-A)/cos(X+A)=1
tan(X-A)=1

then try from there for a simpler approach?
Reply 5
wizz_kid
how?

if u rearrange the equation u get : sin(X-A) - cos(X+A) = 0

divide both sides by cos(X+A)
Reply 6
muster666
I just thought maybe I should write:
sin(X-A)/cos(X+A)=1
tan(X-A)=1

then try from there for a simpler approach?

scrap that i didn't notice it's cos(X+A) not X-A
Reply 7
muster666
sin(X-A)/cos(X+A)=1
sinXcosA-cosXsinA/cosXcosA-sinXsinA=1
(sinXcosA-cosXsinA/cosAcosX)/(cosXcosA-sinXsinA/cosAcosX)=1
(sinX/cosX-sinA/cosA)/1-tanXtanA=1
tanX-tanA/1-tanXtanA=1
tanX-tanA=1-tanXtanA

This is what I've got so far but i have no idea where to go from here


If you keep the sin(x-A) and cos(x+A) on opposite sides of the equation it makes it simpler

sinxcosAcosxsinA=cosxcosAsinxsinA\sin x \cos A - \cos x \sin A = \cos x \cos A - \sin x \sin A

From here, rearrange so sin x and cos x are on opposite sides, and simplify from there :smile:
Reply 8
sonofdot
If you keep the sin(x-A) and cos(x+A) on opposite sides of the equation it makes it simpler

sinxcosAcosxsinA=cosxcosAsinxsinA\sin x \cos A - \cos x \sin A = \cos x \cos A - \sin x \sin A

From here, rearrange so sin x and cos x are on opposite sides, and simplify from there :smile:

Yeah I know that, the problem is actually simplifying it down now, i'm unsure how to approach it, no matter how the equation is structured. only difference is that i simplified down as a 'fraction over a fraction equals 1' as opposed to what you said. it's the same thing really :confused:
oh god of never cood do maths remebrre gcses so soccertoa i think.
Reply 10
muster666
Yeah I know that, the problem is actually simplifying it down now, i'm unsure how to approach it, no matter how the equation is structured. only difference is that i simplified down as a 'fraction over a fraction equals 1' as opposed to what you said. it's the same thing really :confused:


It is the same thing, but my way is easier to simplify. Rearranging gives:

sinxcosA+sinxsinA=cosxcosA+cosxsinA\sin x \cos A + \sin x \sin A = \cos x \cos A + \cos x \sin A

this can be further simplified to:

sinx(cosA+sinA)=cosx(cosA+sinA)\sin x (\cos A + \sin A) = \cos x (\cos A + \sin A)

Can you see where to go from here?
Reply 11
btw, the book im using doesn't have any answers for proof questions such as this, so if someone could write out the correct solution from start to finish i would be very thankful!
Reply 12
sonofdot
It is the same thing, but my way is easier to simplify. Rearranging gives:

sinxcosA+sinxsinA=cosxcosA+cosxsinA\sin x \cos A + \sin x \sin A = \cos x \cos A + \cos x \sin A

this can be further simplified to:

sinx(cosA+sinA)=cosx(cosA+sinA)\sin x (\cos A + \sin A) = \cos x (\cos A + \sin A)

Can you see where to go from here?

oh yeah I get it now, thanks a lot. I think i just had a mental block and managed to forget basic algebra :p:

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