Hi first time poster here!

I'm working with a shape similar to a sector but having "radii" or sides of different lengths. Something similar to the following

enter image description here

Firstly, what would you call such a shape since the radii are unequal?

Secondly, I know angles A (30º) and C (90º) and the length of the curved side 'b' (1º/110.6 km), but I need to find side 'a'. I've been trying to get around this using spherical trigonometric equation:

sinb = tanacotA

But I'm not convinced this is the right approach as only one side of my shape is spherical.

I've also tried using the formula for arc length (a = angle at centre/360 * 2*Pi*R) to find the radius but once again I'm not sure this is the right approach and I have hit somewhat of a wall.

Any ideas about how to go about this?

  • $\begingroup$ Is the arc $AC$ a circular arc? $\endgroup$
    – MPW
    Oct 17, 2016 at 13:31
  • $\begingroup$ Yes with length of 1º $\endgroup$
    – Geoniette
    Oct 17, 2016 at 13:34
  • $\begingroup$ What isthe wall that you hit? $\endgroup$
    – Qwerty
    Oct 17, 2016 at 13:34
  • $\begingroup$ I need to find the length of 'a' but I am not sure which is the right way to go about it, considering that I have a sector with unequal sides or 'radii' $\endgroup$
    – Geoniette
    Oct 17, 2016 at 13:36

1 Answer 1


To form an angle of 30° with the tangent, $AB$ must be part of a chord subtended by a central angle of 60° (see diagram below), so that your picture cannot be accurate - or possibly you didn't explain exactly your problem.

In the case of my diagram, by the sine law you have $$ {d\over\sin 60°}={r\over\sin119°}, $$ so you can find $d$ as a function of $r$ and subsequently $a=r-d$.

Radius $r$ is given because arc $b$ has a length of 110.6 km: $$ {r}={360\over2\pi}\cdot110.6\ \hbox{km}. $$

enter image description here


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