# Question #99e62

$100 {r}^{2} + 20 r + 1 = \left(10 r + 1\right) \left(10 r + 1\right)$
You factor this by looking for combination of integers which when added equal 20 and when multiplied equal 100. Additionally, these integers can be the coefficients of the $r$ variable. A good combination is 10 and 10 giving:
$100 {r}^{2} + 20 r + 1 = \left(10 r + 1\right) \left(10 r + 1\right)$