Wolfram Alpha looks very attractive tool for simple plots. It would be very useful when by copying single line from some manual or calculation example would provide graphical result. In that case it would require ability to vary input constants to obtain the result for certain realization. Engineers often do not care about precise form of expression (which may be rather complicated) but simply put initial values for the result. However I cannot find the way to define constants in Wolfram Alpha command line.

For example; this is a valid line in Wolfram Alpha:

F(l)= 2pil^2; l=0 to 10; plot F

The target line is (which is not working because a and b cannot be constants):

a=1; b=2; F(l)= (b-a)/(-a+3b/4)pi*l^2; l=0 to 10; plot F

Values a and b could be easily modified without changing rest of the command line (expression and other commands). Actually it is bit strange that line above is not working.

It is possible to place all values directly (working):

F(l)= (2-1)/(-1+32/4)pi*l^2; l=0 to 10; plot F

But this is not a good way for "easy way" persons especially when numbers and expressions goes complicated. Searching for all places where you need to place multiple numbers directly would often cause typing errors.

So, is there EASY way to define constants in Wolfram Alpha command line?