The figure is missing an important step - where the ‘a’ variable is eliminated from the third row.
There are two steps.
- In the left image, multiply the first row by 2 and subtract → replace on the third row. This removes the ‘a’ variable from the 3rd row.
- Now Multiply the 2nd row by some value and subtract the 3rd row. This removes the ‘b’ variable from the 3rd row.
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