Express 3−3x−2x23 - 3x - 2x^2 in the form a(x+b)2+ca(x + b)^2 + c and hence state the coordinates of the turning point of the curve y=3−3x−2x2y = 3 - 3x - 2x^2.

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Worked solution

3

1. Factor out the coefficient of $x^2$ from the $x$ terms

3−3x−2x2=−2(x2+32x)+33 - 3x - 2x^2 = -2\left(x^2 + \frac{3}{2}x\right) + 3

2. Complete the square for the expression inside the bracket

=−2[(x+34)2−(34)2]+3= -2\left[ \left(x + \frac{3}{4}\right)^2 - \left(\frac{3}{4}\right)^2 \right] + 3
=−2[(x+34)2−916]+3= -2\left[ \left(x + \frac{3}{4}\right)^2 - \frac{9}{16} \right] + 3

3. Expand and simplify to reach the required form

=−2(x+34)2+2(916)+3= -2\left(x + \frac{3}{4}\right)^2 + 2\left(\frac{9}{16}\right) + 3
=−2(x+34)2+98+248= -2\left(x + \frac{3}{4}\right)^2 + \frac{9}{8} + \frac{24}{8}
=−2(x+34)2+338= -2\left(x + \frac{3}{4}\right)^2 + \frac{33}{8}

4. State the coordinates of the turning point

The turning point occurs when x+34=0  ⟹  x=−34x + \frac{3}{4} = 0 \implies x = -\frac{3}{4}.
The corresponding yy-value is 338\frac{33}{8}.
Coordinates of the turning point are (−34,338)\left(-\frac{3}{4}, \frac{33}{8}\right).

Answer

−2(x+34)2+338-2\left(x + \frac{3}{4}\right)^2 + \frac{33}{8}, Turning point: (−34,338)\left(-\frac{3}{4}, \frac{33}{8}\right)

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