The diagram shows part of the curve y=1x2y = \frac{1}{x^2}.

  1. (i)

    Find the area bounded by the curve y=1x2y = \frac{1}{x^2}, the lines x=1x = 1, x=6x = 6 and the xx-axis.

  2. (ii)

    Show that 122+132+142+152+162<56\frac{1}{2^2} + \frac{1}{3^2} + \frac{1}{4^2} + \frac{1}{5^2} + \frac{1}{6^2} < \frac{5}{6}.

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Worked solution

9(i)

1. Set up the definite integral for the area

Area=∫161x2 dx\text{Area} = \int_{1}^{6} \frac{1}{x^2} \, dx

2. Integrate the function

∫16x−2 dx=[x−1−1]16=[−1x]16\int_{1}^{6} x^{-2} \, dx = \left[ \frac{x^{-1}}{-1} \right]_{1}^{6} = \left[ -\frac{1}{x} \right]_{1}^{6}

3. Evaluate the definite integral

(−16)−(−11)=−16+1=56\left( -\frac{1}{6} \right) - \left( -\frac{1}{1} \right) = -\frac{1}{6} + 1 = \frac{5}{6}

Answer

56\frac{5}{6}

9(ii)

1. Calculate the area under the curve from part (i)

Area =∫16x−2 dx=[−1x]16=−16−(−1)=56= \int_{1}^{6} x^{-2} \, dx = \left[ -\frac{1}{x} \right]_{1}^{6} = -\frac{1}{6} - (-1) = \frac{5}{6}

2. Relate the given sum to the area of rectangles under the curve

The sum 122+132+142+152+162\frac{1}{2^2} + \frac{1}{3^2} + \frac{1}{4^2} + \frac{1}{5^2} + \frac{1}{6^2} represents the total area of 5 rectangles, each of width 1, with heights corresponding to the yy-values of the curve at x=2,3,4,5,6x = 2, 3, 4, 5, 6.
Since the curve y=1x2y = \frac{1}{x^2} is decreasing, these rectangles lie entirely below the curve for the interval x=1x=1 to x=6x=6.

3. Conclude the inequality

Therefore, the sum of the areas of these rectangles is strictly less than the area under the curve from x=1x=1 to x=6x=6.
122+132+142+152+162<∫161x2 dx\frac{1}{2^2} + \frac{1}{3^2} + \frac{1}{4^2} + \frac{1}{5^2} + \frac{1}{6^2} < \int_{1}^{6} \frac{1}{x^2} \, dx
122+132+142+152+162<56\frac{1}{2^2} + \frac{1}{3^2} + \frac{1}{4^2} + \frac{1}{5^2} + \frac{1}{6^2} < \frac{5}{6}

Answer

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