Why doesn't my Python program work?

I'm a new programmer, and I decided to pick up Python as my first language.

So I decided to program something where I type in what I want and the program will come up with something. This is a work in progress, and is constantly being improved. However, I ran into an error with a section of my program. I decided to program something that could find the side of a right triangle, given the other sides. So let me show it to you.

tri = raw_input("What side do you need? ")

if tri == "a":
    def a(b, c):

        b = float(b)
        c = float(c)

        if c-b >= 0:
            a = (c-b)*1/2

            print "a is equal to %s" % a

        else:
            print "This triangle doesn't exist!" 

    print a(raw_input("Ok, what is b? "), raw_input("And what is c? "))

elif tri == "b":
    def b(a, c):

        a = float(a)
        c = float(c)

        if c-a >= 0:
            b = (c-a)*1/2

            print "b is equal to %s" % b

        else:
            print "This triangle doesn't exist!" 

    print a(raw_input("Ok, what is a? ", raw_input("And what is c? "))

elif tri == "c":
    def c(a, b):

        a = float(a)
        b = float(b)

        if a+b > 0:
            c = (a+b)*1/2

            print "c is equal to %s" % c

        else:
            print "This triangle doesn't exist!" 

    print c(raw_input("Ok, what is a? ", raw_input("And what is b? "))

 else:
    print "You must choose a valid side of a triangle!"

So apparently everything is fine, but the the second elif (the one to find c, given a and b) turns up a syntax error. I have no idea why it is wrong, since it appears to be consist with the if elif else format. Can someone help me find the problem and fix it?