Solve -10x^2+30x-20

$$- 10 x ^ { 2 } + 30 x - 20$$

Factor

$$10\left(1-x\right)\left(x-2\right)$$

Solution Steps

Factor out 10.

$$10\left(-x^{2}+3x-2\right)$$

Consider -x^{2}+3x-2. Factor the expression by grouping. First, the expression needs to be rewritten as -x^{2}+ax+bx-2. To find a and b, set up a system to be solved.

$$a+b=3 ab=-\left(-2\right)=2$$

Since ab is positive, a and b have the same sign. Since a+b is positive, a and b are both positive. The only such pair is the system solution.

$$a=2 b=1$$

Rewrite -x^{2}+3x-2 as \left(-x^{2}+2x\right)+\left(x-2\right).

$$\left(-x^{2}+2x\right)+\left(x-2\right)$$

Factor out -x in -x^{2}+2x.

$$-x\left(x-2\right)+x-2$$

Factor out common term x-2 by using distributive property.

$$\left(x-2\right)\left(-x+1\right)$$

Rewrite the complete factored expression.

$$10\left(x-2\right)\left(-x+1\right)$$

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.

$$-10x^{2}+30x-20=0$$

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.

$$x=\frac{-30±\sqrt{30^{2}-4\left(-10\right)\left(-20\right)}}{2\left(-10\right)}$$

Square 30.

$$x=\frac{-30±\sqrt{900-4\left(-10\right)\left(-20\right)}}{2\left(-10\right)}$$

Multiply -4 times -10.

$$x=\frac{-30±\sqrt{900+40\left(-20\right)}}{2\left(-10\right)}$$

Multiply 40 times -20.

$$x=\frac{-30±\sqrt{900-800}}{2\left(-10\right)}$$

Add 900 to -800.

$$x=\frac{-30±\sqrt{100}}{2\left(-10\right)}$$

Take the square root of 100.

$$x=\frac{-30±10}{2\left(-10\right)}$$

Multiply 2 times -10.

$$x=\frac{-30±10}{-20}$$

Now solve the equation x=\frac{-30±10}{-20} when ± is plus. Add -30 to 10.

$$x=-\frac{20}{-20}$$

Divide -20 by -20.

$$x=1$$

Now solve the equation x=\frac{-30±10}{-20} when ± is minus. Subtract 10 from -30.

$$x=-\frac{40}{-20}$$

Divide -40 by -20.

$$x=2$$

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

$$-10x^{2}+30x-20=-10\left(x-1\right)\left(x-2\right)$$

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.

$$x ^ 2 -3x +2 = 0$$

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

$$r + s = 3 rs = 2$$

Two numbers r and s sum up to 3 exactly when the average of the two numbers is \frac{1}{2}*3 = \frac{3}{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>

$$r = \frac{3}{2} - u s = \frac{3}{2} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = 2

$$(\frac{3}{2} - u) (\frac{3}{2} + u) = 2$$

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

$$\frac{9}{4} - u^2 = 2$$

Simplify the expression by subtracting \frac{9}{4} on both sides

$$-u^2 = 2-\frac{9}{4} = -\frac{1}{4}$$

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

$$u^2 = \frac{1}{4} u = \pm\sqrt{\frac{1}{4}} = \pm \frac{1}{2}$$

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

$$r =\frac{3}{2} - \frac{1}{2} = 1 s = \frac{3}{2} + \frac{1}{2} = 2$$

Evaluate

$$10\left(1-x\right)\left(x-2\right)$$