document.write( "Question 1200712: What is the answer to thus quadratic inequality. 3(x^2-1)>-8x. The solution should be written in interval notation. \n" ); document.write( "
Algebra.Com's Answer #834915 by math_tutor2020(3817)![]() ![]() ![]() You can put this solution on YOUR website! \n" ); document.write( "Answer: (-∞, -3) U (1/3, ∞)\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "======================================================================\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "Explanation:\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "Let's get everything to one side \n" ); document.write( "3(x^2-1) > -8x \n" ); document.write( "3x^2-3 > -8x \n" ); document.write( "3x^2-3 + 8x > 0 \n" ); document.write( "3x^2 + 8x - 3 > 0\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "To solve that inequality, we'll consider the corresponding equation \n" ); document.write( "3x^2 + 8x - 3 = 0 \n" ); document.write( "We need to find the roots or x intercepts.\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "Factoring may or may not be possible. \n" ); document.write( "The trial-and-error factoring approach is something I'm not fond of, so I prefer the quadratic formula instead. \n" ); document.write( "Plug in: \n" ); document.write( "a = 3 \n" ); document.write( "b = 8 \n" ); document.write( "c = -3 \n" ); document.write( " \n" ); document.write( " \n" ); document.write( "\n" ); document.write( " \n" ); document.write( " \n" ); document.write( "\n" ); document.write( " \n" ); document.write( " \n" ); document.write( "\n" ); document.write( " \n" ); document.write( " \n" ); document.write( "\n" ); document.write( " \n" ); document.write( " \n" ); document.write( "\n" ); document.write( " \n" ); document.write( " \n" ); document.write( "\n" ); document.write( " \n" ); document.write( " \n" ); document.write( "\n" ); document.write( " \n" ); document.write( "Because each root is rational, it turns out that we could have factored previously. \n" ); document.write( "The x = 1/3 leads to 3x = 1 and further leads to 3x-1 = 0 \n" ); document.write( "The x = -3 leads to x+3 = 0 \n" ); document.write( "We have the factors (3x-1) and (x+3) \n" ); document.write( "Therefore, 3x^2 + 8x - 3 = (3x-1)(x+3) \n" ); document.write( "You can use the FOIL rule on (3x-1)(x+3) to get 3x^2 + 8x - 3 again.\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "Anyways, the roots we found were: \n" ); document.write( "x = -3 \n" ); document.write( "x = 1/3\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "Draw out a number line. \n" ); document.write( "Mark -3 on it, and also mark 1/3 \n" ); document.write( "We'll label 3 regions \n" ); document.write( "Region A = stuff to the left of -3 \n" ); document.write( "Region B = stuff between the number -3 and the number 1/3 \n" ); document.write( "Region C = stuff to the right of 1/3 \n" ); document.write( " \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "Pick a value from region A to test. \n" ); document.write( "I'll go for x = -4 \n" ); document.write( "3(x^2-1) > -8x \n" ); document.write( "3((-4)^2-1) > -8(-4) \n" ); document.write( "3(16-1) > 32 \n" ); document.write( "3(15) > 32 \n" ); document.write( "45 > 32 \n" ); document.write( "The final result is a true statement. \n" ); document.write( "Therefore, 3(x^2-1)>-8x is true when x = -4 \n" ); document.write( "Furthermore, 3(x^2-1)>-8x is true for any value in region A. \n" ); document.write( "We can write x < -3 as part of the solution set\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "Now let's test region B. \n" ); document.write( "I'll use x = 0 \n" ); document.write( "3(x^2-1) > -8x \n" ); document.write( "3(0^2-1) > -8*0 \n" ); document.write( "3(0-1) > 0 \n" ); document.write( "3(-1) > 0 \n" ); document.write( "-3 > 0 \n" ); document.write( "That is false, so region B is crossed off the list.\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "Lastly we need to test region C. \n" ); document.write( "I'll pick x = 2. \n" ); document.write( "3(x^2-1) > -8x \n" ); document.write( "3(2^2-1) > -8*2 \n" ); document.write( "3(4-1) > -16 \n" ); document.write( "3(3) > -16 \n" ); document.write( "9 > -16 \n" ); document.write( "This is true, which makes x > 1/3 also part of the solution set.\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "Our solution set consists of x values such that \n" ); document.write( "x < -3 or x > 1/3\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "We can rewrite x < -3 as -∞ < x < -3 \n" ); document.write( "We can rewrite x > 1/3 as 1/3 < x < ∞ \n" ); document.write( "Both of these help us get toward interval notation.\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "-∞ < x < -3 in interval notation is (-∞, -3) \n" ); document.write( "1/3 < x < ∞ in interval notation is (1/3, ∞)\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "Those disjoint intervals are glued together with the union operator to arrive at the final answer of (-∞, -3) U (1/3, ∞)\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "This is what the solution set looks like when graphed on a number line: \n" ); document.write( " \n" ); document.write( "Take note of the open holes at -3 and at 1/3. \n" ); document.write( "Verbally we can describe the graph as having \"open holes at -3 and at 1/3, with shading everywhere but between those open holes\".\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "An alternative graph to plot is y = 3x^2+8x-3 \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "The parabola is above the x axis when either x < -3 or when x > 1/3. \n" ); document.write( "In other words, the parabola is on or below the x axis when \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "Desmos and GeoGebra are two graphing options I recommend. \n" ); document.write( "The graphing option allows a person to quickly arrive at the solution set. \n" ); document.write( "However, I recommend following an algebraic approach and then use a graph to verify (rather than solely rely on a graph to do all the work for you).\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "A similar problem is found here\r \n" ); document.write( " \n" ); document.write( "\n" ); document.write( "See this article for further reading. \n" ); document.write( " \n" ); document.write( " \n" ); document.write( " |