Definition
Doubles fact
A doubles fact adds a number to another copy of itself.
Continue from this idea into connected AXIO math.
Boundary check: The doubles fact is 9 + 9, even though 2 × 9 has the same value.
What it means
It adds a number to another copy of itself, such as 8 + 8. Because both parts are the same, a doubles fact can be a strong mental anchor for a near double such as 8 + 9.
Remember it
Same number + same number = a double.
Example
9 + 9 = 18 is a doubles fact because both addends are 9. It can help with 9 + 10, which is one more: 19.
Watch for
2 × 9 = 18 has the same value, but the doubles fact is the addition statement 9 + 9 = 18.
Common mix-up
Near doubles have addends one apart, so 8 + 9 is not itself a doubles fact.
Exact meaning
In a doubles fact, one addend is repeated, so the addition has the form a + a for some number a.