article II. Each state retains its sovereignty, freedom and independence, and every power, jurisdiction and right, which is not by this confederation expressly delegated to the united states, in congress assembled.
Article III. the said states herby severally enter into a firm league of friendship with each other, for their common defence , the security of their liberties, and their mutual and general welfare, binding themselves to assist each other, against all force offered to, or attacks made upon them, or any of them, on account of religion, sovereignty, trade, or any other pretence whatever...
the united states in congress assembled, shall never engage in a war.. nor enter into any treaties or alliances, nor coin money... nor borrow money on the credit of the united states, nor appropriate money, nor agree upon the number of vessels of war to be built or purchased, or the number of land or sea forces to be raised, nor appoint a commander in chief of the army or navy, unless nine states assent approve to the same, nor shall a question on any other point, except for adjourning from day to day, be determined unless by the votes of a majority of the united states in congress assembled

how is the principle of the social contract reflected in the declaration of independence or the articles of confederation?

The Social Contract principle states that man was naturally free, and lived without rules or order, but in order to protect their rights and properties government was created. Under the Social Contract theory, government is only valid with the consent of people. This principle inspired the Founding Fathers and can be found under the Articles of Confederation and the Declaration of Independence.

Thomas Jefferson reflected the Social Contract theory into the Declaration of Independence by stating that Britain's Crown had not fulfilled their duties as Government of the Colonies and that this was cause of separation. ("Governments are instituted among Men, deriving their just powers from the consent of the governed").

The Articles of Confederation reflected the Social Contract theory by maintaining the sovereignty of states (Article I) and creating a union where each state gave consent to the central government to use its power. The idea of Article II that nine states had to agree so that the central government could act, is also a reflection of the social contract.