Exercise 2.6.2. Specify the 2 by 2 matrix that has the following effects:

- projecting all vectors onto the axis
- projecting the resulting vectors onto the axis

Answer: From the discussion in the first part of section 2.6 we know that the matrix

will project vectors onto the axis.

Similarly the matrix

will project vectors onto the axis.

To accomplish both operations in succession we therefore need to multiply vectors by the first matrix and then by the second; this corresponds to multiplying vectors by the product matrix

Note that the resulting matrix projects all vectors onto the origin.

NOTE: This continues a series of posts containing worked out exercises from the (out of print) book Linear Algebra and Its Applications, Third Edition by Gilbert Strang.

If you find these posts useful I encourage you to also check out the more current Linear Algebra and Its Applications, Fourth Edition, Dr Strang’s introductory textbook Introduction to Linear Algebra, Fourth Edition and the accompanying free online course, and Dr Strang’s other books.