r^2-40r+400=0

Solution for r^2-40r+400=0 equation:

Simplifying
r2 + -40r + 400 = 0

Reorder the terms:
400 + -40r + r2 = 0

Solving
400 + -40r + r2 = 0

Solving for variable 'r'.

Factor a trinomial.
(20 + -1r)(20 + -1r) = 0

Subproblem 1
Set the factor '(20 + -1r)' equal to zero and attempt to solve:

Simplifying
20 + -1r = 0

Solving
20 + -1r = 0

Move all terms containing r to the left, all other terms to the right.

Add '-20' to each side of the equation.
20 + -20 + -1r = 0 + -20

Combine like terms: 20 + -20 = 0
0 + -1r = 0 + -20
-1r = 0 + -20

Combine like terms: 0 + -20 = -20
-1r = -20

Divide each side by '-1'.
r = 20

Simplifying
r = 20
Subproblem 2
Set the factor '(20 + -1r)' equal to zero and attempt to solve:

Simplifying
20 + -1r = 0

Solving
20 + -1r = 0

Move all terms containing r to the left, all other terms to the right.

Add '-20' to each side of the equation.
20 + -20 + -1r = 0 + -20

Combine like terms: 20 + -20 = 0
0 + -1r = 0 + -20
-1r = 0 + -20

Combine like terms: 0 + -20 = -20
-1r = -20

Divide each side by '-1'.
r = 20

Simplifying
r = 20
Solution
r = {20, 20}