$1.$Consider a basic application that calculates the total cost of items in a shopping cart. The application allows users to add items to the cart, remove items, and retrieve the total cost. Describe two unit tests that you would write to ensure the functionality of this application.$$ Each test should have a description, test steps, and expected output.

$2.$Explain why test$-$driven development is a crucial factor in successfully designing and deploying a software application.$$ In your explanation, explain how you plan to use the principles of test$-$driven development within your Library Management System project.

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Cite the sources you used, if any.