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Question
Find the range of values for which x^2 +2x is greater than or equal to 8.

Answer
Probably the best thing to do is first plot the function, y = x^2 +2x, to get an idea of what you are dealing with. You'll see that it is bowl-shaped with one minimum at x = -1.

Clearly, the function grows larger as x moves away from -1 (both in the positive and negative directions) and will exceed 8 at two values of x. To find these values, it makes sense to find out for which values of x the function y equals 8. Another way to say this is to find the "roots" of the equation obtained by subtracting the number 8 from both sides of the equation for y above,

f(x) = y - 8 =

x^2 +2x - 8 = 0.

It is pretty easy to see that this equation "factors" into f(x) = (x+4)(x-2) = 0, from which it can be seen immediately that x = -4 and x = 2 solve this equation, that is, if you plug in either x=-4 or x= 2, then f(x) = 0. These values of x are called the roots of f(x).  (You could have also used the quadratic equation to obtain these values).

So now we know the values of x for which y = 8. From the plot of the function, it is easy to see that if x > 2 or x < -4, then y > 8, which is the answer.

Randy

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randy patton

Expertise

college mathematics, applied math, advanced calculus, complex analysis, linear and abstract algebra, probability theory, signal processing, undergraduate physics, physical oceanography

Experience

26 years as a professional scientist conducting academic quality research on mostly classified projects involving math/physics modeling and simulation, data analysis and signal processing, instrument development; often ocean related

Publications
J. Physical Oceanography, 1984 "A Numerical Model for Low-Frequency Equatorial Dynamics", with M. Cane

Education/Credentials
M.S. MIT Physical Oceanography, B.S. UC Berkeley Applied Math

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Also an Expert in Oceanography

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