BigDecimal and BigInteger do not provide any methods to do roots or "to the power of" operations.
There are no exact root or "to the power of" operations available on primitives, either. There are no root or "to the power of" operations at all on integers. They do exist for "float" and "double", but are approximate. Of course, they couldn't be anything else, because "float" and "double" are always approximate. Even if you set a "double" variable to 9.0, it may not actually be set to exactly nine, and its square-root will not come out as exactly three.
Computers generally implement powers via an approximation (polynomial series, or something like that). Therefore, the "to the power of" operation is perhaps not appropriate for BigDecimal or BigInteger, which only do exact operations.
You should convert your BigDecimal to a "double", use the appropriate Math methods on it, and convert it back, remembering that the result is approximate.