# equation of semi circle

• Sep 22nd 2008, 05:13 PM
arturdo968
equation of semi circle
what is the equation of a semi circle with:

-diameter of 6
-amplitude of 3
-starting at the x-axis
-the center is at (5,0)

and what is the generic equation of a semi circle?
thank you
• Sep 22nd 2008, 05:30 PM
11rdc11
Put your info into the standard equation of a circle

$\frac{(x-5)^2}{9} + \frac{y^2}{9} =1$

and then solve for y and that will give you the equation to a semi circle

$y = \pm \sqrt{9-(x-5)^2}$
• Sep 22nd 2008, 05:34 PM
arturdo968
what is the first formula that you showed? i'm not familiar with it
• Sep 22nd 2008, 05:41 PM
11rdc11
$(x-a)^2 + (y-b)^2 = r^2$

a and being your center points respectively (5,0)
• Sep 22nd 2008, 05:50 PM
arturdo968
oh ok i see it now. sorry, thank you very much though, that helped me out. do you by anychance know how to set restrictions on lines so that it only graphs a segment?
• Sep 22nd 2008, 06:23 PM
11rdc11
Nope but when you find out let me know to please. Still haven't figured out how to do it on my TI89
• Sep 22nd 2008, 06:30 PM
arturdo968
that's exactly what i'm trying to figure out. i'm trying to find the way to set up a domain and range on it and i still can't lol. i'll make sure to tell you if i find something. this is for a project due thursday =p
• Sep 23rd 2008, 10:01 AM
bkarpuz
In parametric form, you may let $\cos(t)=\frac{x-5}{3}$ and $\sin(t)=\frac{y}{3}$ to get $\alpha(t)=(x(t),y(t))=(3\cos(t)+5,\sin(t))$ for $t\in[0,\pi]$.