The catenary curve (also called a chainette, alysoid, funicular, see wiki) - the curve of hanging chain or cable or other fibre in a homogenous gravitational field - is used in many design tasks. To create a sketch shapes as a catenary in Inventor you can zse the function "Equation curve" and specify the curve parametrically.
On the above linked wiki page you can find the generalized equation of a planar catenary curve:
y = a * cosh (x / a)
where a is the parameter of uniform scaling of the curve.
Start the function Equation curve (Sketch > Create > Line > Equation curve):
and enter the equation directly to the equation fieleds - choose the equation option "Explicit" and "Cartesian". Just make sure the correct units are combined - i.e. convert the length value of "x" to angle (mm->rad). Using the value of a = 1.8 and the range x -2 to 2:
y = 1.8 * cosh (x / 1 mm * 1 rad / 1.8)
For possible subsequent editing of the curve, you the function "Edit equation curve" from the context menu in the model browser tree.