This is the problem with math education in America.
- Not the students who hand in this mistake.
- Not the teachers nationwide who cringe at this mistake.
- Not the schools who do their best with unwilling or unmotivated students.
No, it's the college teacher preparation programs that instill faulty knowledge and reinforce it with extra credit ... these students obviously don't know their subject all that well and this "teacher" is no better. "Hands On Math: Burn The Textbooks, Shred The Worksheets, Teach Math." is the blog motto.
Seems like reading a book and learning a fact or two might come in handy.