Solve f(x)=-x^2-4x+1

$$f ( x ) = - x ^ { 2 } - 4 x + 1$$

Factor

$$-\left(x-\left(-\sqrt{5}-2\right)\right)\left(x-\left(\sqrt{5}-2\right)\right)$$

Steps Using the Quadratic Formula

Quadratic polynomial can be factored using the transformation ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right), where x_{1} and x_{2} are the solutions of the quadratic equation ax^{2}+bx+c=0.

$$-x^{2}-4x+1=0$$

All equations of the form ax^{2}+bx+c=0 can be solved using the quadratic formula: \frac{-b±\sqrt{b^{2}-4ac}}{2a}. The quadratic formula gives two solutions, one when ± is addition and one when it is subtraction.

$$x=\frac{-\left(-4\right)±\sqrt{\left(-4\right)^{2}-4\left(-1\right)}}{2\left(-1\right)}$$

Square -4.

$$x=\frac{-\left(-4\right)±\sqrt{16-4\left(-1\right)}}{2\left(-1\right)}$$

Multiply -4 times -1.

$$x=\frac{-\left(-4\right)±\sqrt{16+4}}{2\left(-1\right)}$$

Add 16 to 4.

$$x=\frac{-\left(-4\right)±\sqrt{20}}{2\left(-1\right)}$$

Take the square root of 20.

$$x=\frac{-\left(-4\right)±2\sqrt{5}}{2\left(-1\right)}$$

The opposite of -4 is 4.

$$x=\frac{4±2\sqrt{5}}{2\left(-1\right)}$$

Multiply 2 times -1.

$$x=\frac{4±2\sqrt{5}}{-2}$$

Now solve the equation x=\frac{4±2\sqrt{5}}{-2} when ± is plus. Add 4 to 2\sqrt{5}.

$$x=\frac{2\sqrt{5}+4}{-2}$$

Divide 4+2\sqrt{5} by -2.

$$x=-\left(\sqrt{5}+2\right)$$

Now solve the equation x=\frac{4±2\sqrt{5}}{-2} when ± is minus. Subtract 2\sqrt{5} from 4.

$$x=\frac{4-2\sqrt{5}}{-2}$$

Divide 4-2\sqrt{5} by -2.

$$x=\sqrt{5}-2$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute -\left(2+\sqrt{5}\right) for x_{1} and -2+\sqrt{5} for x_{2}.

$$-x^{2}-4x+1=-\left(x-\left(-\left(\sqrt{5}+2\right)\right)\right)\left(x-\left(\sqrt{5}-2\right)\right)$$

Steps Using Direct Factoring Method

Quadratic equations such as this one can be solved by a new direct factoring method that does not require guess work. To use the direct factoring method, the equation must be in the form x^2+Bx+C=0.

$$x ^ 2 +4x -1 = 0$$

Let r and s be the factors for the quadratic equation such that x^2+Bx+C=(x−r)(x−s) where sum of factors (r+s)=−B and the product of factors rs = C

$$r + s = -4 rs = -1$$

Two numbers r and s sum up to -4 exactly when the average of the two numbers is \frac{1}{2}*-4 = -2. You can also see that the midpoint of r and s corresponds to the axis of symmetry of the parabola represented by the quadratic equation y=x^2+Bx+C. The values of r and s are equidistant from the center by an unknown quantity u. Express r and s with respect to variable u. <div style='padding: 8px'><img src='https://opalmath.azureedge.net/customsolver/quadraticgraph.png' style='width: 100%;max-width: 700px' /></div>

$$r = -2 - u s = -2 + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -1

$$(-2 - u) (-2 + u) = -1$$

Simplify by expanding (a -b) (a + b) = a^2 – b^2

$$4 - u^2 = -1$$

Simplify the expression by subtracting 4 on both sides

$$-u^2 = -1-4 = -5$$

Simplify the expression by multiplying -1 on both sides and take the square root to obtain the value of unknown variable u

$$u^2 = 5 u = \pm\sqrt{5} = \pm \sqrt{5}$$

The factors r and s are the solutions to the quadratic equation. Substitute the value of u to compute the r and s.

$$r =-2 - \sqrt{5} = -4.236 s = -2 + \sqrt{5} = 0.236$$

Evaluate

$$1-4x-x^{2}$$