Solve 4.9x^2-41.6x+102=0

$$4.9 x ^ { 2 } - 41.6 x + 102 = 0$$

Solve for x (complex solution)

$$x=\frac{208+i\times 2\sqrt{1679}}{49}\approx 4.244897959+1.672473571i$$

$$x=\frac{-i\times 2\sqrt{1679}+208}{49}\approx 4.244897959-1.672473571i$$

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.

$$4.9x^{2}-41.6x+102=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 4.9 for a, -41.6 for b, and 102 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-\left(-41.6\right)±\sqrt{\left(-41.6\right)^{2}-4\times 4.9\times 102}}{2\times 4.9}$$

Square -41.6 by squaring both the numerator and the denominator of the fraction.

$$x=\frac{-\left(-41.6\right)±\sqrt{1730.56-4\times 4.9\times 102}}{2\times 4.9}$$

Multiply -4 times 4.9.

$$x=\frac{-\left(-41.6\right)±\sqrt{1730.56-19.6\times 102}}{2\times 4.9}$$

Multiply -19.6 times 102.

$$x=\frac{-\left(-41.6\right)±\sqrt{1730.56-1999.2}}{2\times 4.9}$$

Add 1730.56 to -1999.2 by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x=\frac{-\left(-41.6\right)±\sqrt{-268.64}}{2\times 4.9}$$

Take the square root of -268.64.

$$x=\frac{-\left(-41.6\right)±\frac{2\sqrt{1679}i}{5}}{2\times 4.9}$$

The opposite of -41.6 is 41.6.

$$x=\frac{41.6±\frac{2\sqrt{1679}i}{5}}{2\times 4.9}$$

Multiply 2 times 4.9.

$$x=\frac{41.6±\frac{2\sqrt{1679}i}{5}}{9.8}$$

Now solve the equation x=\frac{41.6±\frac{2\sqrt{1679}i}{5}}{9.8} when ± is plus. Add 41.6 to \frac{2i\sqrt{1679}}{5}.

$$x=\frac{208+2\sqrt{1679}i}{5\times 9.8}$$

Divide \frac{208+2i\sqrt{1679}}{5} by 9.8 by multiplying \frac{208+2i\sqrt{1679}}{5} by the reciprocal of 9.8.

$$x=\frac{208+2\sqrt{1679}i}{49}$$

Now solve the equation x=\frac{41.6±\frac{2\sqrt{1679}i}{5}}{9.8} when ± is minus. Subtract \frac{2i\sqrt{1679}}{5} from 41.6.

$$x=\frac{-2\sqrt{1679}i+208}{5\times 9.8}$$

Divide \frac{208-2i\sqrt{1679}}{5} by 9.8 by multiplying \frac{208-2i\sqrt{1679}}{5} by the reciprocal of 9.8.

$$x=\frac{-2\sqrt{1679}i+208}{49}$$

The equation is now solved.

$$x=\frac{208+2\sqrt{1679}i}{49} x=\frac{-2\sqrt{1679}i+208}{49}$$

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.

$$4.9x^{2}-41.6x+102=0$$

Subtract 102 from both sides of the equation.

$$4.9x^{2}-41.6x+102-102=-102$$

Subtracting 102 from itself leaves 0.

$$4.9x^{2}-41.6x=-102$$

Divide both sides of the equation by 4.9, which is the same as multiplying both sides by the reciprocal of the fraction.

$$\frac{4.9x^{2}-41.6x}{4.9}=-\frac{102}{4.9}$$

Dividing by 4.9 undoes the multiplication by 4.9.

$$x^{2}+\left(-\frac{41.6}{4.9}\right)x=-\frac{102}{4.9}$$

Divide -41.6 by 4.9 by multiplying -41.6 by the reciprocal of 4.9.

$$x^{2}-\frac{416}{49}x=-\frac{102}{4.9}$$

Divide -102 by 4.9 by multiplying -102 by the reciprocal of 4.9.

$$x^{2}-\frac{416}{49}x=-\frac{1020}{49}$$

Divide -\frac{416}{49}, the coefficient of the x term, by 2 to get -\frac{208}{49}. Then add the square of -\frac{208}{49} to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}-\frac{416}{49}x+\left(-\frac{208}{49}\right)^{2}=-\frac{1020}{49}+\left(-\frac{208}{49}\right)^{2}$$

Square -\frac{208}{49} by squaring both the numerator and the denominator of the fraction.

$$x^{2}-\frac{416}{49}x+\frac{43264}{2401}=-\frac{1020}{49}+\frac{43264}{2401}$$

Add -\frac{1020}{49} to \frac{43264}{2401} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x^{2}-\frac{416}{49}x+\frac{43264}{2401}=-\frac{6716}{2401}$$

Factor x^{2}-\frac{416}{49}x+\frac{43264}{2401}. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x-\frac{208}{49}\right)^{2}=-\frac{6716}{2401}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-\frac{208}{49}\right)^{2}}=\sqrt{-\frac{6716}{2401}}$$

Simplify.

$$x-\frac{208}{49}=\frac{2\sqrt{1679}i}{49} x-\frac{208}{49}=-\frac{2\sqrt{1679}i}{49}$$

Add \frac{208}{49} to both sides of the equation.

$$x=\frac{208+2\sqrt{1679}i}{49} x=\frac{-2\sqrt{1679}i+208}{49}$$