将 x2−6x+18x^{2} - 6x + 18 表示为 (x−p)2+q(x - p)^{2} + q 的形式。 由此,在下方坐标轴中画出 y=x2−6x+18y = x^{2} - 6x + 18 的图像草图。清晰地标出拐点(顶点)以及与坐标轴的所有交点。

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完整解法

2(a)

1. Complete the square and simplify the remaining constant.

x2−6x+18=(x−3)2−9+18=(x−3)2+9x^{2}-6x+18=(x-3)^{2}-9+18=(x-3)^{2}+9

答案

x2−6x+18=(x−3)2+9x^{2}-6x+18=(x-3)^{2}+9

2(b)

1. Use the completed-square form to identify the turning point and shape of the parabola.

From part (a),
y=x2−6x+18=(x−3)2+9.y=x^2-6x+18=(x-3)^2+9.
Since the coefficient of (x−3)2(x-3)^2 is positive, the graph opens upwards.

Turning point: (3,9)(3,9).

2. Find and state the axial intercepts, then describe the required sketch.

For the yy-intercept, put x=0x=0:
y=02−6(0)+18=18.y=0^2-6(0)+18=18.
So the yy-intercept is (0,18)(0,18).

For the xx-intercepts, put y=0y=0:
0=(x−3)2+9.0=(x-3)^2+9.
This has no real solution since (x−3)2≥0(x-3)^2\geq 0, so (x−3)2+9>0(x-3)^2+9>0 for all real xx.

Therefore, the graph lies above the xx-axis and has no xx-intercepts.

答案

Sketch an upward-opening parabola with turning point (3,9)(3,9), crossing the yy-axis at (0,18)(0,18), and with no xx-intercepts.

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