Solve 20t-25t^2=4

$$20 t - 25 t ^ { 2 } = 4$$

Solve for t

$$t=\frac{2}{5}=0.4$$

Steps Using Factoring By Grouping

Subtract 4 from both sides.

$$20t-25t^{2}-4=0$$

Rearrange the polynomial to put it in standard form. Place the terms in order from highest to lowest power.

$$-25t^{2}+20t-4=0$$

To solve the equation, factor the left hand side by grouping. First, left hand side needs to be rewritten as -25t^{2}+at+bt-4. To find a and b, set up a system to be solved.

$$a+b=20 ab=-25\left(-4\right)=100$$

Since ab is positive, a and b have the same sign. Since a+b is positive, a and b are both positive. List all such integer pairs that give product 100.

$$1,100 2,50 4,25 5,20 10,10$$

Calculate the sum for each pair.

$$1+100=101 2+50=52 4+25=29 5+20=25 10+10=20$$

The solution is the pair that gives sum 20.

$$a=10 b=10$$

Rewrite -25t^{2}+20t-4 as \left(-25t^{2}+10t\right)+\left(10t-4\right).

$$\left(-25t^{2}+10t\right)+\left(10t-4\right)$$

Factor out -5t in the first and 2 in the second group.

$$-5t\left(5t-2\right)+2\left(5t-2\right)$$

Factor out common term 5t-2 by using distributive property.

$$\left(5t-2\right)\left(-5t+2\right)$$

To find equation solutions, solve 5t-2=0 and -5t+2=0.

$$t=\frac{2}{5} t=\frac{2}{5}$$

Steps Using the Quadratic Formula

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.

$$-25t^{2}+20t=4$$

Subtract 4 from both sides of the equation.

$$-25t^{2}+20t-4=4-4$$

Subtracting 4 from itself leaves 0.

$$-25t^{2}+20t-4=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute -25 for a, 20 for b, and -4 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$t=\frac{-20±\sqrt{20^{2}-4\left(-25\right)\left(-4\right)}}{2\left(-25\right)}$$

Square 20.

$$t=\frac{-20±\sqrt{400-4\left(-25\right)\left(-4\right)}}{2\left(-25\right)}$$

Multiply -4 times -25.

$$t=\frac{-20±\sqrt{400+100\left(-4\right)}}{2\left(-25\right)}$$

Multiply 100 times -4.

$$t=\frac{-20±\sqrt{400-400}}{2\left(-25\right)}$$

Add 400 to -400.

$$t=\frac{-20±\sqrt{0}}{2\left(-25\right)}$$

Take the square root of 0.

$$t=-\frac{20}{2\left(-25\right)}$$

Multiply 2 times -25.

$$t=-\frac{20}{-50}$$

Reduce the fraction \frac{-20}{-50} to lowest terms by extracting and canceling out 10.

$$t=\frac{2}{5}$$

Steps for Completing the Square

Quadratic equations such as this one can be solved by completing the square. In order to complete the square, the equation must first be in the form x^{2}+bx=c.

$$-25t^{2}+20t=4$$

Divide both sides by -25.

$$\frac{-25t^{2}+20t}{-25}=\frac{4}{-25}$$

Dividing by -25 undoes the multiplication by -25.

$$t^{2}+\frac{20}{-25}t=\frac{4}{-25}$$

Reduce the fraction \frac{20}{-25} to lowest terms by extracting and canceling out 5.

$$t^{2}-\frac{4}{5}t=\frac{4}{-25}$$

Divide 4 by -25.

$$t^{2}-\frac{4}{5}t=-\frac{4}{25}$$

Divide -\frac{4}{5}, the coefficient of the x term, by 2 to get -\frac{2}{5}. Then add the square of -\frac{2}{5} to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$t^{2}-\frac{4}{5}t+\left(-\frac{2}{5}\right)^{2}=-\frac{4}{25}+\left(-\frac{2}{5}\right)^{2}$$

Square -\frac{2}{5} by squaring both the numerator and the denominator of the fraction.

$$t^{2}-\frac{4}{5}t+\frac{4}{25}=\frac{-4+4}{25}$$

Add -\frac{4}{25} to \frac{4}{25} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$t^{2}-\frac{4}{5}t+\frac{4}{25}=0$$

Factor t^{2}-\frac{4}{5}t+\frac{4}{25}. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(t-\frac{2}{5}\right)^{2}=0$$

Take the square root of both sides of the equation.

$$\sqrt{\left(t-\frac{2}{5}\right)^{2}}=\sqrt{0}$$

Simplify.

$$t-\frac{2}{5}=0 t-\frac{2}{5}=0$$

Add \frac{2}{5} to both sides of the equation.

$$t=\frac{2}{5} t=\frac{2}{5}$$

The equation is now solved. Solutions are the same.

$$t=\frac{2}{5}$$