[plt-scheme] (no subject)

Hi guys, I am a freshman student in Kenya. After I stumbled on MIT's ocw
website, I got interested in Scheme and after a few while on this LtU site,
I felt like starting with DrScheme is the best way since its tailoured to
the biginners. I am very new in Programming and we just don't have great
teachers who have command of the subject here in Kenya.
Well after installing and running DrScheme, I encountered a problem which I
will be very greatful to anyone of you who can help.
I difined this procedure: (define (area-of-disk r)
(* 3.14 (* r r)))
which worked when I ran it like this (area-of-disk 5)

then I defined this: (define (area-of-ring outer inner)
(- (area-of-disk outer)
(area-of-disk inner)))
However upon running this procedure I got this message; area-of-disk: name
is not defined, not an argument, and not a primitive name.
and this on the tracing window: No tracing results are available, yet. (Make
sure that your language supports tracing and that tracing is enabled.)
Can anyone help?


-- 
Live, Learn, Love!
Patrick Amkayah.
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