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.
$$2.5t^{2}+30t-160=0$$
This equation is in standard form: $ax^{2}+bx+c=0$. Substitute $2.5$ for $a$, $30$ for $b$, and $-160$ for $c$ in the quadratic formula, $\frac{-b±\sqrt{b^{2}-4ac}}{2a}$.
Now solve the equation $t=\frac{-30±50}{5}$ when $±$ is plus. Add $-30$ to $50$.
$$t=\frac{20}{5}$$
Divide $20$ by $5$.
$$t=4$$
Now solve the equation $t=\frac{-30±50}{5}$ when $±$ is minus. Subtract $50$ from $-30$.
$$t=-\frac{80}{5}$$
Divide $-80$ by $5$.
$$t=-16$$
The equation is now solved.
$$t=4$$ $$t=-16$$
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 $x^{2}+bx=c$.
Divide both sides of the equation by $2.5$, which is the same as multiplying both sides by the reciprocal of the fraction.
$$\frac{2.5t^{2}+30t}{2.5}=\frac{160}{2.5}$$
Dividing by $2.5$ undoes the multiplication by $2.5$.
$$t^{2}+\frac{30}{2.5}t=\frac{160}{2.5}$$
Divide $30$ by $2.5$ by multiplying $30$ by the reciprocal of $2.5$.
$$t^{2}+12t=\frac{160}{2.5}$$
Divide $160$ by $2.5$ by multiplying $160$ by the reciprocal of $2.5$.
$$t^{2}+12t=64$$
Divide $12$, the coefficient of the $x$ term, by $2$ to get $6$. Then add the square of $6$ to both sides of the equation. This step makes the left hand side of the equation a perfect square.
$$t^{2}+12t+6^{2}=64+6^{2}$$
Square $6$.
$$t^{2}+12t+36=64+36$$
Add $64$ to $36$.
$$t^{2}+12t+36=100$$
Factor $t^{2}+12t+36$. In general, when $x^{2}+bx+c$ is a perfect square, it can always be factored as $\left(x+\frac{b}{2}\right)^{2}$.
$$\left(t+6\right)^{2}=100$$
Take the square root of both sides of the equation.
