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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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Question 7

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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.

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Answer

To find the total area of the two shaded squares, we define the area in terms of A. The square on the left has sides of length A, and the square on the right has sides of length (10 - A).

Thus, the total area can be expressed as: Area=A2+(10A)2\text{Area} = A^2 + (10 - A)^2

Expanding this expression: Area=A2+(10020A+A2)=2A220A+100\text{Area} = A^2 + (100 - 20A + A^2) = 2A^2 - 20A + 100

Next, we find the minimum possible value of this quadratic function. The vertex of a parabola represented by (ax^2 + bx + c) occurs at (A = -\frac{b}{2a}). Here, (a = 2) and (b = -20).

Calculating the vertex position: A=202×2=5A = -\frac{-20}{2 \times 2} = 5

Substituting A = 5 back into the area function to find the minimum area: Area=2(52)20(5)+100=50\n(minimum area)\text{Area} = 2(5^2) - 20(5) + 100 = 50\n\text{(minimum area)}

Thus, the minimum possible value of the total area 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

Using the Pythagorean theorem, we can analyze the right triangle formed at the bottom right of the square, where the lengths of the legs are A and (10 - A).

The length of the diagonal, d, is given by: d=A2+(10A)2d = \sqrt{A^2 + (10 - A)^2}

Expanding the expression for d, we have: d=A2+(10020A+A2)=2A220A+100d = \sqrt{A^2 + (100 - 20A + A^2)} = \sqrt{2A^2 - 20A + 100}

Now, let's notice that: d2=2A220A+100d^2 = 2A^2 - 20A + 100

This follows from our previous equation for the area, which shows: d2=Aread^2 = \text{Area}

Therefore, it is verified that the value of the total area of the two shaded squares is indeed equal to d².

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