20=a(a-1)

Solution for 20=a(a-1) equation:

Simplifying
20 = a(a + -1)

Reorder the terms:
20 = a(-1 + a)
20 = (-1 * a + a * a)
20 = (-1a + a2)

Solving
20 = -1a + a2

Solving for variable 'a'.

Reorder the terms:
20 + a + -1a2 = -1a + a + a2 + -1a2

Combine like terms: -1a + a = 0
20 + a + -1a2 = 0 + a2 + -1a2
20 + a + -1a2 = a2 + -1a2

Combine like terms: a2 + -1a2 = 0
20 + a + -1a2 = 0

Factor a trinomial.
(5 + -1a)(4 + a) = 0

Subproblem 1
Set the factor '(5 + -1a)' equal to zero and attempt to solve:

Simplifying
5 + -1a = 0

Solving
5 + -1a = 0

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

Add '-5' to each side of the equation.
5 + -5 + -1a = 0 + -5

Combine like terms: 5 + -5 = 0
0 + -1a = 0 + -5
-1a = 0 + -5

Combine like terms: 0 + -5 = -5
-1a = -5

Divide each side by '-1'.
a = 5

Simplifying
a = 5
Subproblem 2
Set the factor '(4 + a)' equal to zero and attempt to solve:

Simplifying
4 + a = 0

Solving
4 + a = 0

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

Add '-4' to each side of the equation.
4 + -4 + a = 0 + -4

Combine like terms: 4 + -4 = 0
0 + a = 0 + -4
a = 0 + -4

Combine like terms: 0 + -4 = -4
a = -4

Simplifying
a = -4
Solution
a = {5, -4}