Chord Length: A Mathematical Observation

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The discussion centers around calculating chord length using the formula 2r sin(θ/2). Participants emphasize the need to first determine the angle θ from the arc length using the relationship s = rθ. One participant calculates θ as 1.7 from the given arc length of 3.4 and radius of 2. They then apply the law of cosines to find the chord length, ultimately confirming that the correct expression involves sin(0.85). The conversation highlights the importance of deriving θ before applying the chord length formula.
karush
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Ok this should be just an observation solution ..
But isn't the equation for chord length
$$2r\sin{\frac{\theta}{2}}=
\textit{chord length}$$

Don't see any of the options
Derived from that..
 

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Hi karush.

All the information is there. You just need to calculate the angle $\theta$ from the arc length $s=r\theta$.
 
Olinguito said:
Hi karush.

All the information is there. You just need to calculate the angle $\theta$ from the arc length $s=r\theta$.


It is asking for chord lenght!
 
I would start with the arc-length formula to find the subtended angle:

$$\theta=\frac{s}{r}=\frac{3.4}{2}=1.7$$

Then, use the law of cosines:

$$\overline{DF}=\sqrt{2^2+2^2-2(2)(2)\cos(1.7)}=2\sqrt{2-2\cos(1.7)}$$

Lastly, a double-angle identity for cosine:

$$\overline{DF}=2\sqrt{4\sin^2(0.85)}=4\sin(0.85)$$
 
Ok
So that's where .85 comes from
So then it's D
 
karush said:
It is asking for chord lenght!

And to do that, you need to know the angle $\theta$, don’t you? Calculate that first!
 
Why of course we do!

However for this SAT question
It is only asking which
Expression to use
We should know that 1.7 is not the $\theta$ we need
and we have use sin $\theta$
So even without any calculations we should see that it is D
 
Last edited:
You were asking
karush said:
Don't see any of the options
That was because you were given arc length $s=3.4$ (and radius $r=2$) but not $\theta$. I was therefore instructing you to compute $\theta$ from the formula $s=r\theta$ so you could use it in the formula $2r\sin\dfrac{\theta}2$ for the chord length.
 

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