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Express 2x² + 11x + 6 in the form a(x+b)² + c and find the minimum value.

Mathematics

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Answer

y=2(x+114)2−738y = 2\left(x + \frac{11}{4}\right)^2 - \frac{73}{8}

−738-\frac{73}{8}

Step-by-step solution

5 steps

To express y=2x2+11x+6y = 2x^2 + 11x + 6 in the form a(x+b)2+ca(x+b)^2 + c and find its minimum value, we use the method of completing the square.

  1. Factor out the coefficient of x2x^2 from the terms involving xx.

    y=2(x2+112x)+6y = 2(x^2 + \frac{11}{2}x) + 6
  2. Complete the square inside the parenthesis.

    To do this, take half of the coefficient of xx (112\frac{11}{2}), square it, and add and subtract it within the parenthesis.
    Half of 112\frac{11}{2} is 114\frac{11}{4}.
    Squaring 114\frac{11}{4} gives (114)2=12116(\frac{11}{4})^2 = \frac{121}{16}.

    y=2(x2+112x+12116−12116)+6y = 2\left(x^2 + \frac{11}{2}x + \frac{121}{16} - \frac{121}{16}\right) + 6
  3. Group the perfect square trinomial and distribute the 2.

    y=2((x+114)2−12116)+6y = 2\left(\left(x + \frac{11}{4}\right)^2 - \frac{121}{16}\right) + 6 y=2(x+114)2−2(12116)+6y = 2\left(x + \frac{11}{4}\right)^2 - 2\left(\frac{121}{16}\right) + 6 y=2(x+114)2−1218+6y = 2\left(x + \frac{11}{4}\right)^2 - \frac{121}{8} + 6
  4. Combine the constant terms.

    y=2(x+114)2−1218+488y = 2\left(x + \frac{11}{4}\right)^2 - \frac{121}{8} + \frac{48}{8} y=2(x+114)2−738y = 2\left(x + \frac{11}{4}\right)^2 - \frac{73}{8}

    This is in the form a(x+b)2+ca(x+b)^2 + c, where a=2a=2, b=114b=\frac{11}{4}, and c=−738c=-\frac{73}{8}.

  5. Find the minimum value.

    Since a=2a=2 is positive, the parabola opens upwards, and the vertex represents the minimum point. The minimum value of the expression a(x+b)2+ca(x+b)^2 + c occurs when (x+b)2=0(x+b)^2 = 0, which means x=−bx = -b. At this point, the value of yy is cc.
    The minimum value is −738-\frac{73}{8}.

    The expression in the form a(x+b)2+ca(x+b)^2 + c is:

    y=2(x+114)2−738\boxed{y = 2\left(x + \frac{11}{4}\right)^2 - \frac{73}{8}}

    The minimum value is:

    −738\boxed{-\frac{73}{8}}
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