Geometry Question (1 Viewer)

[Bi0mic]

Hey all -

Just going through some past papers and came across 1998 6b(iii). Unfortunately i cant scan it and it comes with an image,making it hard for anyone other than people that have the paper to help me out here!...

Anyway, 6biii asks to find the angle DOE in radians - i used the basic arc/angle relation (@=lr) and got 0.32 radians. However, the answers use the cosine rule to find the angle, which turns out to be 0.319. Same answer right? but the question asks to correct to 3 decimal places.

What im asking here is, is my process viable in an exam and perhaps ill just round my 0.32 answer down a decimal place, or would it be worth it to just do the triangle/cosine method rather than the quick @=rl formulae. If this is the case, why is it that they dont ask to use a certain process as although my answer is correct, it can be seen as a shortcut.

Thanks all (Y)

 

[CrashOveride]

that wasn't 98

 

[Bi0mic]

arr dam 1999 ... im holding the past paper book in front of me lol .. how sad

 

[mojako]

well I havent seen the paper... but I can download it, but I prefer to try my deductive skills first

reading from ur description and how they used cosine rule, it seems that they gave u the chord length, not the arc length.

also, it's l=r@ ==> @=l/r, not @=lr
but if u used @=lr, your answer would be significantly different than the given answer, regardless of whether they give chord or arc length. so u used @=l/r and typed the wrong thing on this forum.

also, if it's true that they gave the chord length and u used @=l/r, l is the chord length... you should get smaller @ than the given answer... but you get it larger.. so maybe they gave the arc length.. but if it's arc length we can't use cosine rule.

so maybe you made two mistakes:
1. misinterpret chord length as arc length
2. rounding error or broken calculator

thats my hypothesis

EDIT: well from Xayma's post I know there are at least two circles involved. But I won't bother revising my hypothesis (which assumed 1 circle was involved).

 

[Jase]

I did that same thing you did. However, see, what they ask for is the answer to 3 d.p.
So even though 3.2 is right, you cant write 3.200. you won't get the marks.
Anyway i thought using the cosine rule was pretty stupid when you can just use the arc length... they just want to make things difficult for a 2 mark question.

 

[Xayma]

No I go back to my original I think the arc length is different in both cases OED arc length ED is different to FED arc length ED. But it might not be (cant entirely remember).

In any case they round off for the answer of the chord length, which would mean the true answer might be yours.