x2/a2+y2/b2=1

If you mean

x^2 / a^2   + y^2 / b^2 = 1

This is the graph of an ellipse .....  (assuming a ≠ b)...... centered at the origin......with its vertices located at (±a, 0) and (0, ±b)

If a = b, we just have a circle centered at the origin with  a = b = r

x2/a2+y2/b2=1

the meaning is not clear.

x^2 =  $${{\mathtt{x}}}^{{\mathtt{2}}}$$

or

x_2    =  $$x_2$$