excel in the formula,

In D1, type = sqrt (A1 ^ 2-4 * B1 * C1), when the A1 ^ 2 <4 * B1 * C1, it will show # num
I would like to, enter if (A1 ^ 2> 4 * B1 * C1, sqrt (A1 ^ 2-4 * B1 * C1), sqrt (A1 ^ 2 +4 * B1 * C1)) should be no problem.
But in fact, A1B1C1 cell formulas are very Long before the definition of the original formula is also very much if we join the definition too much.
Can not function can not be used to deal with it?
For example, if (Not (# num), sqrt (A1 ^ 2 — 4 * B1 * C1), sqrt (A1 ^ 2 +4 * B1 * C1))

drag formula of previous coverage You can look at the possibility of this?
= SQRT (A1 ^ 2 +4 * B1 * C1 * IF (A1 ^ 2> 4 * B1 * C1, -1,1))