# calculating a point on circumference

See the diagram

Known values are

A: (-87.91, 41.98)
B: (-104.67, 39.85)
C: (-96.29, 40.92)
L: 14.63  // L is OC


Known angles

ADB: 60 deg


How to calculate Point F? that is 10 deg from point A.

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L is OC or OE ? – Bhargav Jan 3 '12 at 17:15
Homework assignment? – Hidde Jan 3 '12 at 18:19

Given that two of the angles in $\triangle ABO$ have measure $60°$, $\triangle ABO$ is equilateral. It appears that $C$ is the midpoint of $\overline{AB}$, so $L$ is the length of an altitude of $\triangle ABO$ and the lengths of the sides of the triangle are $\frac{2}{\sqrt{3}}L\approx16.89$. Now, $O$ is $16.89$ from both $A$ and $B$, which gives a system of equations that can be solved for the coordinates of $O$: $(-94.4456,26.4022)$ (as shown in your picture, so I'll use this one) or $(-98.1344,55.4278)$ (which would be above $\overline{AB}$).

Assuming that the arc shown is intended to be circular and centered at $O$, $F$ is the image of $A$ under a $10°$ rotation about $O$, which we can carry out by applying to $A$:

1. a translation that takes $O$ to $(0,0)$, $(x,y)\to(x+98.1344,y-55.4278)$,
2. a rotation of $10°$ about the origin, $(x,y)\to(x\cos10°-y\sin10°,y\cos10°+x\sin10°)$, and
3. a translation that takes $(0,0)$ to $O$, $(x,y)\to(x-98.1344,y+55.4278)$.

Carrying out these transformations on $A$ gives $$F\approx(-90.7144, 42.8782).$$

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if L is OE then u have the radius,

OA= L

AF = $\frac{\theta }{2\pi }2\pi r$ where r=L

u know A,O

so solve the 2 equations to get coordinates of F

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sorry! L is OC. but I can calculate R using R = tan(30) x L – coure2011 Jan 3 '12 at 17:32
can you please explain how to calculate F? I need Fx and Fy! – coure2011 Jan 3 '12 at 17:42

C is exactly the center of AB, so COA = 30 deg. AOF is 10 deg (given), so EOF is 20 deg.

Calculate R using tan(30)•L. Calculate d(F,EO) using R•sin(20).

The entire shape is turned, so turn everything so that OE and the y-axis are parallel.

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