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A square with sides of length 10 units is shown in the diagram - Junior Cycle Mathematics - Question 7 - 2011

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A square with sides of length 10 units is shown in the diagram. A point A is chosen on a diagonal of the square, and two shaded squares are constructed as shown. By ... show full transcript

Worked Solution & Example Answer:A square with sides of length 10 units is shown in the diagram - Junior Cycle Mathematics - Question 7 - 2011

Step 1

Find the minimum possible value of the total area of the two shaded squares. Justify your answer fully.

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Answer

To find the total area of the two shaded squares, we denote the length of the side of the square at point A as A. The square to the left has sides of length A, and the square to the right has sides of length (10 - A).

The formulas for the areas of the squares are:

  • Area of the left square: A2A^2
  • Area of the right square: (10A)2(10 - A)^2

Thus, the total area will be:

extTotalArea=A2+(10A)2=A2+10020A+A2=2A220A+100 ext{Total Area} = A^2 + (10 - A)^2 = A^2 + 100 - 20A + A^2 = 2A^2 - 20A + 100

To minimize this quadratic function, we can complete the square or take the derivative and set it to zero:

  1. Finding the vertex of the parabola: The vertex form is given by: A = - rac{b}{2a} = - rac{-20}{2 imes 2} = 5

  2. Substituting A back into the area equation:

    Total Area when A = 5: extTotalArea=2(5)220(5)+100=50 ext{Total Area} = 2(5)^2 - 20(5) + 100 = 50

  3. Conclusion: Therefore, the minimum possible value of the total area of the two shaded squares is 50 square units.

Step 2

Show that the value of the total area of the two shaded squares is equal to d².

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Answer

Given the overall shape is a square, the triangle formed in the bottom right corner must be a right-angled triangle. This triangle has sides of length A and (10 - A), with hypotenuse d.

By applying the Pythagorean theorem, we have:

d2=A2+(10A)2d^2 = A^2 + (10 - A)^2

Now, expanding the equation: d2=A2+(10A)2=A2+(10020A+A2)=2A220A+100d^2 = A^2 + (10 - A)^2 = A^2 + (100 - 20A + A^2) = 2A^2 - 20A + 100

Since we previously calculated the total area of the two shaded squares, we can observe that: extTotalArea=d2 ext{Total Area} = d^2

Thus, we can conclude that the value of the total area of the two shaded squares is indeed equal to d².

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