Solve t^2-17t+70

$$t ^ { 2 } - 17 t + 70$$

Factor

$$\left(t-10\right)\left(t-7\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as t^{2}+at+bt+70. To find a and b, set up a system to be solved.

$$a+b=-17 ab=1\times 70=70$$

Since ab is positive, a and b have the same sign. Since a+b is negative, a and b are both negative. List all such integer pairs that give product 70.

$$-1,-70 -2,-35 -5,-14 -7,-10$$

Calculate the sum for each pair.

$$-1-70=-71 -2-35=-37 -5-14=-19 -7-10=-17$$

The solution is the pair that gives sum -17.

$$a=-10 b=-7$$

Rewrite t^{2}-17t+70 as \left(t^{2}-10t\right)+\left(-7t+70\right).

$$\left(t^{2}-10t\right)+\left(-7t+70\right)$$

Factor out t in the first and -7 in the second group.

$$t\left(t-10\right)-7\left(t-10\right)$$

Factor out common term t-10 by using distributive property.

$$\left(t-10\right)\left(t-7\right)$$

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.

$$t^{2}-17t+70=0$$

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.

$$t=\frac{-\left(-17\right)±\sqrt{\left(-17\right)^{2}-4\times 70}}{2}$$

Square -17.

$$t=\frac{-\left(-17\right)±\sqrt{289-4\times 70}}{2}$$

Multiply -4 times 70.

$$t=\frac{-\left(-17\right)±\sqrt{289-280}}{2}$$

Add 289 to -280.

$$t=\frac{-\left(-17\right)±\sqrt{9}}{2}$$

Take the square root of 9.

$$t=\frac{-\left(-17\right)±3}{2}$$

The opposite of -17 is 17.

$$t=\frac{17±3}{2}$$

Now solve the equation t=\frac{17±3}{2} when ± is plus. Add 17 to 3.

$$t=\frac{20}{2}$$

Divide 20 by 2.

$$t=10$$

Now solve the equation t=\frac{17±3}{2} when ± is minus. Subtract 3 from 17.

$$t=\frac{14}{2}$$

Divide 14 by 2.

$$t=7$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute 10 for x_{1} and 7 for x_{2}.

$$t^{2}-17t+70=\left(t-10\right)\left(t-7\right)$$

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.

$$x ^ 2 -17x +70 = 0$$

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

$$r + s = 17 rs = 70$$

Two numbers r and s sum up to 17 exactly when the average of the two numbers is \frac{1}{2}*17 = \frac{17}{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>

$$r = \frac{17}{2} - u s = \frac{17}{2} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = 70

$$(\frac{17}{2} - u) (\frac{17}{2} + u) = 70$$

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

$$\frac{289}{4} - u^2 = 70$$

Simplify the expression by subtracting \frac{289}{4} on both sides

$$-u^2 = 70-\frac{289}{4} = -\frac{9}{4}$$

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

$$u^2 = \frac{9}{4} u = \pm\sqrt{\frac{9}{4}} = \pm \frac{3}{2}$$

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

$$r =\frac{17}{2} - \frac{3}{2} = 7 s = \frac{17}{2} + \frac{3}{2} = 10$$

Evaluate

$$\left(t-10\right)\left(t-7\right)$$