Whyd i lose a mark for this? (1 Viewer)

[kkk579]

dont u just sin both sides then prove that RHS=-x by using the identity f(finverse(x)) = x o

 

[iloveeggs]

you probably lost the mark bc you manipulated the equation before proving. when you are given a show or prove question you have to take 1 side and manipulate that side only to produce the other side

 

[iloveeggs]

bc its a one marker i think they expect you to prove that arcsinx is an odd function by drawing a rough graph of it

 

[kkk579]

you probably lost the mark bc you manipulated the equation before proving. when you are given a show or prove question you have to take 1 side and manipulate that side only to produce the other side

but how else would u do it - this is ngo and sons taught it so i js did the same thing

 

[kkk579]

bc its a one marker i think they expect you to prove that arcsinx is an odd function by drawing a rough graph of it

its only one mark why do they want a graph as well

 

[iloveeggs]

just draw a rough version. i was a bit extra with the f(x) stuff, you could just skip that too and just do the graph, say arcsinx is odd and go straight to the answer

 

[kkk579]

just draw a rough version. i was a bit extra with the f(x) stuff, you could just skip that too and just do the graph, say arcsinx is odd and go straight to the answer

Yeah this makes sense too but idk i just did it the way my tutor did

 

[iloveeggs]

Yeah this makes sense too but idk i just did it the way my tutor did

are you sure they told you to do that? bc thats crazy and not acceptable in the hsc

you literally never manipulate an identity or equation you are supposed to prove, you only use one side and do stuff to it to equate it to the other side. that's like you rewriting the question in a way you know how to answer it

 

[iloveeggs]

let me try to do it your way that makes more sense

 

[Average Boreduser]

its only one mark why do they want a graph as well

It’s a proof qn. U can’t manipulate. That’s only if u have a show than qn.

 

[iloveeggs]

i used the same identity as you, i just didnt edit the initial equation they gave us and proves from lhs to rhs

 

[iloveeggs]

hopefully that helped, you can use the identity your tutor gave you but you can never change the question

 

[kkk579]

hopefully that helped, you can use the identity your tutor gave you but you can never change the question

yeah ur proof makes sense thanks!
but f(f inverse (x)) is just an identity u learn in functions so its not like made up by my tutor or anything, what I meant was like i copied the exact proof my tutor wrote because the question was the exact same. i just didnt use the triangle method and used sin(-x)=-sinx but the concept of moving the sin over is used the same

 

[iloveeggs]

yeah ur proof makes sense thanks!
but f(f inverse (x)) is just an identity u learn in functions so its not like made up by my tutor or anything, what I meant was like i copied the exact proof my tutor wrote because the question was the exact same. i just didnt use the triangle method and used sin(-x)=-sinx but the concept of moving the sin over is used the same

okay so this image that you have attached here just showed only proves that -x=sin(-arcsinx). technically speaking you can solve it this way and then say arcsin(-x)=-arcsin(x) but youll get no marks for it in the hsc realistically if its a 1 marker, maybe a pity mark if its a 2 marker. here is the reason:

the only reason they arrived at -x=sin(-arcsinx) is bc they are taking the thing you are required to prove which is arcsin(-x)=-arcsin(x) and then taking the arcsin over to the other side. that is the only way they know found that identity, they didnt use any other identities to get there. you are not allowed to manipulate the equation that you are supposed to prove. if they asked you to solve an equation you would do that, but not proving. this is one of the worst ways to answer this question bc it shows the marker you dont know how to prove it as it is (which is what they want from you).

here is a part of the syllabus for inverse trig for e1


you can use -x=sin(-arcsinx) to solve the rest of it IF AND ONLY IF you prove -x=sin(-arcsinx) using other identities like sin(arcsinx)=x shown in that syllabus dot point AND then sub it in in some sort of way and still show a clear RHS to LHS proof. refer to the second picture i sent

i dont understand why you are clinging to this method tho. why are so adamant about doing it using that way? clearly you have to memorise the stupid triangle or whatever. the graph is the easier way to do it, it doesnt even have to be that detailed

 

[kkk579]

okay so this image that you have attached here just showed only proves that -x=sin(-arcsinx). technically speaking you can solve it this way and then say arcsin(-x)=-arcsin(x) but youll get no marks for it in the hsc realistically if its a 1 marker, maybe a pity mark if its a 2 marker. here is the reason:

the only reason they arrived at -x=sin(-arcsinx) is bc they are taking the thing you are required to prove which is arcsin(-x)=-arcsin(x) and then taking the arcsin over to the other side. that is the only way they know found that identity, they didnt use any other identities to get there. you are not allowed to manipulate the equation that you are supposed to prove. if they asked you to solve an equation you would do that, but not proving. this is one of the worst ways to answer this question bc it shows the marker you dont know how to prove it as it is (which is what they want from you).

here is a part of the syllabus for inverse trig for e1
View attachment 44298

you can use -x=sin(-arcsinx) to solve the rest of it IF AND ONLY IF you prove -x=sin(-arcsinx) using other identities like sin(arcsinx)=x shown in that syllabus dot point AND then sub it in in some sort of way and still show a clear RHS to LHS proof. refer to the second picture i sent

i dont understand why you are clinging to this method tho. why are so adamant about doing it using that way? clearly you have to memorise the stupid triangle or whatever. the graph is the easier way to do it, it doesnt even have to be that detailed

im clinging to this method so i can guilt trip the teacher into giving me the mark

 

[iloveeggs]

im clinging to this method so i can guilt trip the teacher into giving me the mark

lmao you do you