x squared - i = 0
What symbol(s) is(are) needed to make Delimiters (. . .) bigger to completely encompass fraction?
Either you can just choose bigger ones, like \big( and \big), or \Big( and \Big), or \Big[ and \Big].
Or you can use pairs of \left( and \right) or \left[ and \right] that will be resized automatically to the height of the expression that is contained in them, like this