Final answer:
President Thomas Jefferson justified the purchase of the Louisiana Territory using the elastic / necessary & proper clause of the Constitution.
Step-by-step explanation:
President Thomas Jefferson justified the purchase of the Louisiana Territory from France using the elastic / necessary & proper clause of the Constitution. This clause, also known as the Elastic Clause, is found in Article I, Section 8. It gives Congress the power to make laws that are necessary and proper for carrying out its other powers.
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