Year 12 Further Maths
How to recognise Stretch matrices have zeros in the two positions off the main diagonal
Check that you agree the following, in which a general stretch matrix acts on the column vector (x, y)
Notice that all x-values are now a times bigger, and that y-values are b times bigger. (Of course, you must substitute 'smaller' for 'bigger' if either of a or b are less than 1.)
If a and b are equal, then the stretch is the same in all directions, and we have an enlargement of scale factor a.
Application to combinations of transformations
If you multiply a whole matrix by a stretching matrix, then the whole of the top row gets multiplied by the x-stretch and the whole of the bottom row by the y-stretch
Notice that you can reverse engineer this, and do a kind of factorisation on the right-hand matrix in this equation: X is a factor of both elements in the top row, while Y is a factor of both elements in the bottom row. Under these circumstances you can 'factorise' as shown by the left-hand side of the equation. (Note that if there is no obvious common factor in one or other of the rows, then X or Y will have to be given the value 1 in the left-hand matrix.)