(a) A trustee shall administer the trust solely in the interests of the beneficiaries.
(b) Subject to the rights of persons dealing with or assisting the trustee as provided in section one thousand twelve, article ten of this chapter, a sale, encumbrance or other transaction involving the investment or management of trust property entered into by the trustee for the trustee's own personal account or which is otherwise affected by a conflict between the trustee's fiduciary and personal interests is voidable by a beneficiary affected by the transaction unless:
(1) The transaction was authorized by the terms of the trust instrument;