Check out the fact triangles worksheets on the first grade worksheet page.
Fact triangles do a couple of important things. First, they reinforce a critical first grade objective: memorizing addition facts with sums up to 18. But in addition they also start to reinforce in your first grader the inverse relationship between addition and subtraction.
4 + 5 = 9
5 + 4 = 9
9 – 4 = 5
9 – 5 = 4
You’ll find two types of fact triangles. The first (pictured here) includes all three digits. In the second type the student is provided with two numbers and has to use reasoning to figure out the third.