X

9x

x

_____

It is helpful to know your multiplication tables, so you can find factors that meed certain criteria.

In the first problem, you are looking for factors of 21 that add to 10. The numbers that divide 21 are (1, 21) or (3, 7). Of these, the latter pair add to 10.

The second problem is a standard form: The difference of 2 squares.

The third problem seeks factors of 25 that add to 10. You know they're not 1 and 25, so 5 and 5 is the other (useful) choice.

^{2}-10x+21 = (x-3)(x-7)9x

^{2}-25 = (3x+5)(3x-5)x

^{2}+10x+25 = (x+5)^{2}_____

It is helpful to know your multiplication tables, so you can find factors that meed certain criteria.

In the first problem, you are looking for factors of 21 that add to 10. The numbers that divide 21 are (1, 21) or (3, 7). Of these, the latter pair add to 10.

The second problem is a standard form: The difference of 2 squares.

The third problem seeks factors of 25 that add to 10. You know they're not 1 and 25, so 5 and 5 is the other (useful) choice.