The diagram represents a plot of land, ABCD. The police is looking for a robber hiding in the land. Scale: 1 cm represents 1 km Here are the accounts from three witnesses:
Witness 1: The robber is located more than 6 km away from A.
Witness 2: The robber is located nearer to B than A.
Witness 3: The robber is located nearer to AD than AB.
Shade the region where the robber is hiding.

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Worked solution

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1. Represent the condition “more than 6 km away from A”.

Since the scale is 1 cm:1 km1\text{ cm}:1\text{ km}, 6 km6\text{ km} is represented by 6 cm6\text{ cm}.

Draw a circle with centre AA and radius 6 cm6\text{ cm}.

The robber must be in the region outside this circle.

2. Represent the condition “nearer to B than A”.

Construct the perpendicular bisector of ABAB.

Points on this perpendicular bisector are equally far from AA and BB.

Since the robber is nearer to BB than AA, choose the side of the perpendicular bisector that contains BB.

3. Represent the condition “nearer to AD than AB”.

Construct the angle bisector of ∠DAB\angle DAB.

Points on the angle bisector are equally far from the lines ADAD and ABAB.

Since the robber is nearer to ADAD than ABAB, choose the side of the angle bisector closer to ADAD.

4. Shade the required intersection inside the plot.

Within the boundary of plot ABCDABCD, shade only the region satisfying all three conditions:

outside circle centre A radius 6 cm\text{outside circle centre } A \text{ radius } 6\text{ cm}

∩ side of perpendicular bisector of AB containing B\cap\ \text{side of perpendicular bisector of } AB \text{ containing } B

∩ side of angle bisector of ∠DAB nearer to AD.\cap\ \text{side of angle bisector of } \angle DAB \text{ nearer to } AD.

Answer

Shade the part of plot ABCDABCD that is outside the circle centre AA radius 6 cm6\text{ cm}, on the BB-side of the perpendicular bisector of ABAB, and on the ADAD-side of the angle bisector of ∠DAB\angle DAB.

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