The Principle of Square Roots Let’s consider x 2 = 25. We know that the number 25 has two...
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Transcript of The Principle of Square Roots Let’s consider x 2 = 25. We know that the number 25 has two...
The Principle of Square Roots
Let’s consider x2 = 25. We know that the number 25 has two real-number square roots, 5 and -5, the solutions of the equation. Thus we see that square roots can provide quick solutions for equations of the type x2 = k, where k is a constant.
Example
Solution
Solve (x + 3)2 = 7
2( 3) 7x
3 7 or 3 7x x
The solutions are 3 7.
3 7 or 3 7.x x
2
27
Completing the Square
Not all quadratic equations can be solved as in the previous examples. By using a method called completing the square, we can use the principle of square roots to solve any quadratic equation.
Solve x2 + 10x + 4 = 0
Example
Solution
Solve x2 + 10x + 4 = 0
2( 5) 21x
2 10 4 0x x
2 10 4x x
x2 + 10x + 25 = –4 + 25
5 21 or 5 21x x
5 21. The solutions are
Using the principle of square roots
Factoring
Adding 25 to both sides.
2.591,0 0.257,0
Example
Solution
Jackson invested $5800 at an interest rate of r, compounded annually. In 2 years, it grew to $6765. What was the interest rate?
1. Introduction. We are already familiar with the compound-interest formula.
(1 )tA P r
6765 = 5800(1 + r)2
The translation consists of substituting into the formula:
2. Body. Solve for r:
6765/5800 = (1 + r)2
6765/5800 1 r
1 6765/5800 r
.080 or 2.080r r
Since the interest rate cannot be negative, the solution is .080 or 8.0%.
3. Conclusion. The interest rate was 8.0%.