Question on proof (1 Viewer)

[TheZhangarang]

Have a question regarding proofs using results. On the CSSA Trial Paper this year, I got 0/3 for q15 a.v. (The tan^2x +1 = sec^2x) one. I proved the result by subbing the values into RHS and proving RHS = x1 rather than expanding x1 so that x1=RHS

Technically though, I answered the question using the identity in the process but my teacher said something about not being allowed to prove a result with a result. In the future, am I allowed to prove RHS = LHS rather than LHS = RHS.

 

[InteGrand]

It's pretty stupid if they don't give you the marks just because you did "RHS = LHS rather than LHS = RHS" (but some markers are like this I think unfortunately).

Did you assume the result that needed to be proved in doing your proof? That's something that cannot be done for a valid proof.

 

[TheZhangarang]

it was asking me to prove x = f(y) and in a previous part, we had proved x = 2tana

I simply proved that f(y) = 2tana = x

Is that assuming the result?

 

[InteGrand]

it was asking me to prove x = f(y) and in a previous part, we had proved x = 2tana

I simply proved that f(y) = 2tana = x

Is that assuming the result?

Don't know what method you used to prove f(y) = 2tan(a), but it doesn't look like you assumed the result to do so. So what you did should be correct.

 

[seanieg89]

You should probably post the full question / your proof attempt if you want a completely accurate critique here.

 

[pikachu975]

It didn't ask you to prove tan^2x + 1 = sec^2x, it said USE that identity to prove that x1 = 2h(sqrt(2h/y1 -1))

So it was like:
tanx = x1/2h
cosx = y1/2h

Then just sub those values into the identity it provided to get the answer.