-5t^2+9t+21=0

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Solution for -5t^2+9t+21=0 equation:


Simplifying
-5t2 + 9t + 21 = 0

Reorder the terms:
21 + 9t + -5t2 = 0

Solving
21 + 9t + -5t2 = 0

Solving for variable 't'.

Begin completing the square.  Divide all terms by
-5 the coefficient of the squared term: 

Divide each side by '-5'.
-4.2 + -1.8t + t2 = 0

Move the constant term to the right:

Add '4.2' to each side of the equation.
-4.2 + -1.8t + 4.2 + t2 = 0 + 4.2

Reorder the terms:
-4.2 + 4.2 + -1.8t + t2 = 0 + 4.2

Combine like terms: -4.2 + 4.2 = 0.0
0.0 + -1.8t + t2 = 0 + 4.2
-1.8t + t2 = 0 + 4.2

Combine like terms: 0 + 4.2 = 4.2
-1.8t + t2 = 4.2

The t term is -1.8t.  Take half its coefficient (-0.9).
Square it (0.81) and add it to both sides.

Add '0.81' to each side of the equation.
-1.8t + 0.81 + t2 = 4.2 + 0.81

Reorder the terms:
0.81 + -1.8t + t2 = 4.2 + 0.81

Combine like terms: 4.2 + 0.81 = 5.01
0.81 + -1.8t + t2 = 5.01

Factor a perfect square on the left side:
(t + -0.9)(t + -0.9) = 5.01

Calculate the square root of the right side: 2.238302929

Break this problem into two subproblems by setting 
(t + -0.9) equal to 2.238302929 and -2.238302929.

Subproblem 1

t + -0.9 = 2.238302929 Simplifying t + -0.9 = 2.238302929 Reorder the terms: -0.9 + t = 2.238302929 Solving -0.9 + t = 2.238302929 Solving for variable 't'. Move all terms containing t to the left, all other terms to the right. Add '0.9' to each side of the equation. -0.9 + 0.9 + t = 2.238302929 + 0.9 Combine like terms: -0.9 + 0.9 = 0.0 0.0 + t = 2.238302929 + 0.9 t = 2.238302929 + 0.9 Combine like terms: 2.238302929 + 0.9 = 3.138302929 t = 3.138302929 Simplifying t = 3.138302929

Subproblem 2

t + -0.9 = -2.238302929 Simplifying t + -0.9 = -2.238302929 Reorder the terms: -0.9 + t = -2.238302929 Solving -0.9 + t = -2.238302929 Solving for variable 't'. Move all terms containing t to the left, all other terms to the right. Add '0.9' to each side of the equation. -0.9 + 0.9 + t = -2.238302929 + 0.9 Combine like terms: -0.9 + 0.9 = 0.0 0.0 + t = -2.238302929 + 0.9 t = -2.238302929 + 0.9 Combine like terms: -2.238302929 + 0.9 = -1.338302929 t = -1.338302929 Simplifying t = -1.338302929

Solution

The solution to the problem is based on the solutions from the subproblems. t = {3.138302929, -1.338302929}

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