15. Is it possible for a 4 × 4 matrix to be invertible when its columns do not span R*? Why or why not? 16. If an n x n matrix A is invertible, then the columns of AT are linearly independent. Explain why. 17. Can a square matrix with two identical columns be invert- ible? Why or why not?
Unitary Method
The word “unitary” comes from the word “unit”, which means a single and complete entity. In this method, we find the value of a unit product from the given number of products, and then we solve for the other number of products.
Speed, Time, and Distance
Imagine you and 3 of your friends are planning to go to the playground at 6 in the evening. Your house is one mile away from the playground and one of your friends named Jim must start at 5 pm to reach the playground by walk. The other two friends are 3 miles away.
Profit and Loss
The amount earned or lost on the sale of one or more items is referred to as the profit or loss on that item.
Units and Measurements
Measurements and comparisons are the foundation of science and engineering. We, therefore, need rules that tell us how things are measured and compared. For these measurements and comparisons, we perform certain experiments, and we will need the experiments to set up the devices.
![### Linear Algebra: Advanced Problem Set
**15.** Is it possible for a \( 4 \times 4 \) matrix to be invertible when its columns do not span \( \mathbb{R}^4 \)? Why or why not?
**16.** If an \( n \times n \) matrix \( A \) is invertible, then the columns of \( A^T \) are linearly independent. Explain why.
**17.** Can a square matrix with two identical columns be invertible? Why or why not?
**18.** Can a square matrix with two identical rows be invertible? Why or why not?
**19.** If the columns of a \( 7 \times 7 \) matrix \( D \) are linearly independent, what can be said about the solutions of \( Dx = b \)? Why?
**20.** If \( A \) is a \( 5 \times 5 \) matrix and the equation \( Ax = b \) is consistent for every \( b \) in \( \mathbb{R}^5 \), is it possible that for some \( b \), the equation \( Ax = b \) has more than one solution? Why or why not?
**21.** If the equation \( C u = v \) has more than one solution for some \( v \) in \( \mathbb{R}^n \), can the columns of the \( n \times n \) matrix \( C \) span \( \mathbb{R}^n \)? Why or why not?
**22.** If \( n \times n \) matrices \( E \) and \( F \) have the property that \( EF = I \), then \( E \) and \( F \) commute. Explain why.
**23.** Assume that \( F \) is an \( n \times n \) matrix. If the equation \( F x = y \) is inconsistent for some \( y \) in \( \mathbb{R}^n \), what can you say about the equation \( F x = 0 \)? Why?
**24.** If an \( n \times n \) matrix \( G \) cannot be row reduced to \( I_n \), what can you say about the columns of \( G \)? Why?
**25.** Verify the boxed statement preceding Example 1.
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