How to quickly verify if a matrix is the inverse of another using row-column pairing?
Conditions
- You are given two square matrices of the same dimension.
- You need to verify if their product is the identity matrix.
Reasoning, step by step
- Identify the rows of the first matrix and the columns of the second matrix.
- Select a row and a column .
- Compute the dot product of row and column .
- Check if the result is 1 when , and 0 when .
- Repeat for all combinations to fully verify, or use specific combinations to eliminate options in a multiple-choice setting.
Example
To verify if is the inverse of , check the entry: the second row of is and the second column of is . Their dot product is , which equals 1 because it was the entry of . Checking the entry: the first row of is and the second column of is . Their dot product is , which equals 0 because it was the entry of .
Common misconceptions
- Assuming that checking only the diagonal entries is sufficient to prove a matrix is the inverse, without verifying the off-diagonal entries are zero.
- Believing that having the same set of elements in the candidate matrix automatically makes it the inverse.
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