Solve x^2-10x+21geq0

$$x ^ { 2 } - 10 x + 21 \geq 0$$

Solve for x

$$x\in (-\infty,3]\cup [7,\infty)$$

Steps Using the Quadratic Formula

To solve the inequality, factor the left hand side. 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.

$$x^{2}-10x+21=0$$

All equations of the form ax^{2}+bx+c=0 can be solved using the quadratic formula: \frac{-b±\sqrt{b^{2}-4ac}}{2a}. Substitute 1 for a, -10 for b, and 21 for c in the quadratic formula.

$$x=\frac{-\left(-10\right)±\sqrt{\left(-10\right)^{2}-4\times 1\times 21}}{2}$$

Do the calculations.

$$x=\frac{10±4}{2}$$

Solve the equation x=\frac{10±4}{2} when ± is plus and when ± is minus.

$$x=7 x=3$$

Rewrite the inequality by using the obtained solutions.

$$\left(x-7\right)\left(x-3\right)\geq 0$$

For the product to be ≥0, x-7 and x-3 have to be both ≤0 or both ≥0. Consider the case when x-7 and x-3 are both ≤0.

$$x-7\leq 0 x-3\leq 0$$

The solution satisfying both inequalities is x\leq 3.

$$x\leq 3$$

Consider the case when x-7 and x-3 are both ≥0.

$$x-3\geq 0 x-7\geq 0$$

The solution satisfying both inequalities is x\geq 7.

$$x\geq 7$$

The final solution is the union of the obtained solutions.

$$x\leq 3\text{; }x\geq 7$$