#### Answer

$a^2+10ab+25b^2$

#### Work Step by Step

Use the FOIL method to multiply the terms within the two sets of parentheses. Multiply the first value in the first set of parentheses by the first value in the second set, and then the first value in the first by the second value in the second set. Then multiply the second value in the first set by the first value in the second set, and then the second value in the first set by the second value in the second set. Then combine all the like terms.
$(a+5b)^2$
$(a+5b)(a+5b)$
$a^2+5ab+5ab+25b^2$
$a^2+10ab+25b^2$