1373.88=1200(1+r)(1+r)

Solution for 1373.88=1200(1+r)(1+r) equation:

Simplifying
1373.88 = 1200(1 + r)(1 + r)

Multiply (1 + r) * (1 + r)
1373.88 = 1200(1(1 + r) + r(1 + r))
1373.88 = 1200((1 * 1 + r * 1) + r(1 + r))
1373.88 = 1200((1 + 1r) + r(1 + r))
1373.88 = 1200(1 + 1r + (1 * r + r * r))
1373.88 = 1200(1 + 1r + (1r + r2))

Combine like terms: 1r + 1r = 2r
1373.88 = 1200(1 + 2r + r2)
1373.88 = (1 * 1200 + 2r * 1200 + r2 * 1200)
1373.88 = (1200 + 2400r + 1200r2)

Solving
1373.88 = 1200 + 2400r + 1200r2

Solving for variable 'r'.

Combine like terms: 1373.88 + -1200 = 173.88
173.88 + -2400r + -1200r2 = 1200 + 2400r + 1200r2 + -1200 + -2400r + -1200r2

Reorder the terms:
173.88 + -2400r + -1200r2 = 1200 + -1200 + 2400r + -2400r + 1200r2 + -1200r2

Combine like terms: 1200 + -1200 = 0
173.88 + -2400r + -1200r2 = 0 + 2400r + -2400r + 1200r2 + -1200r2
173.88 + -2400r + -1200r2 = 2400r + -2400r + 1200r2 + -1200r2

Combine like terms: 2400r + -2400r = 0
173.88 + -2400r + -1200r2 = 0 + 1200r2 + -1200r2
173.88 + -2400r + -1200r2 = 1200r2 + -1200r2

Combine like terms: 1200r2 + -1200r2 = 0
173.88 + -2400r + -1200r2 = 0

Begin completing the square.  Divide all terms by
-1200 the coefficient of the squared term: 

Divide each side by '-1200'.
-0.1449 + 2r + r2 = 0

Move the constant term to the right:

Add '0.1449' to each side of the equation.
-0.1449 + 2r + 0.1449 + r2 = 0 + 0.1449

Reorder the terms:
-0.1449 + 0.1449 + 2r + r2 = 0 + 0.1449

Combine like terms: -0.1449 + 0.1449 = 0.0000
0.0000 + 2r + r2 = 0 + 0.1449
2r + r2 = 0 + 0.1449

Combine like terms: 0 + 0.1449 = 0.1449
2r + r2 = 0.1449

The r term is 2r.  Take half its coefficient (1).
Square it (1) and add it to both sides.

Add '1' to each side of the equation.
2r + 1 + r2 = 0.1449 + 1

Reorder the terms:
1 + 2r + r2 = 0.1449 + 1

Combine like terms: 0.1449 + 1 = 1.1449
1 + 2r + r2 = 1.1449

Factor a perfect square on the left side:
(r + 1)(r + 1) = 1.1449

Calculate the square root of the right side: 1.07

Break this problem into two subproblems by setting 
(r + 1) equal to 1.07 and -1.07.

Subproblem 1
r + 1 = 1.07

Simplifying
r + 1 = 1.07

Reorder the terms:
1 + r = 1.07

Solving
1 + r = 1.07

Solving for variable 'r'.

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

Add '-1' to each side of the equation.
1 + -1 + r = 1.07 + -1

Combine like terms: 1 + -1 = 0
0 + r = 1.07 + -1
r = 1.07 + -1

Combine like terms: 1.07 + -1 = 0.07
r = 0.07

Simplifying
r = 0.07

Subproblem 2
r + 1 = -1.07

Simplifying
r + 1 = -1.07

Reorder the terms:
1 + r = -1.07

Solving
1 + r = -1.07

Solving for variable 'r'.

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

Add '-1' to each side of the equation.
1 + -1 + r = -1.07 + -1

Combine like terms: 1 + -1 = 0
0 + r = -1.07 + -1
r = -1.07 + -1

Combine like terms: -1.07 + -1 = -2.07
r = -2.07

Simplifying
r = -2.07

Solution
The solution to the problem is based on the solutions
from the subproblems.
r = {0.07, -2.07}