As a middle-school math teacher, I have seen many students who do understand abstract concepts, and even enjoy learning about things like Pi and the Pythagorean Theorem, but who appear to be unable to do so due to a weak grasp of math fundamentals.
I recommend getting one of these books when you first begin homeschooling and use it as a reference throughout your homeschool journey. Regardless of how long you homeschool, you'll always have doubts and questions about how your child is performing.A scope and sequence book can put your mind at ease.
So what kinds of worksheets should you get? Anything where you feel that your child needs further drill. We often have this notion that worksheets are just for math. This, of course, is not true. While they are excellent tools for reviewing math facts such as the multiplication tables and division facts, they are just as useful for reviewing parts of speech or the states in the union.
However for the most part, when a worksheet is needed to help drill down a procedure, standard, or lesson, its effectiveness can and will vary.
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