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There is a tree between houses of A and B. If the tree leans on A’s House, the tree top rests on his window which is 12 m from ground. If the tree leans on B’s House, the tree top rests on his window which is 9 m from ground. If the height of the tree is 15 m, what is distance between A’s and B’s house?

  • A.21 m
  • B.25 m
  • C.16 m
  • D.12 m

Answer: A

The leaning tree is the hypotenuse of a right triangle in each case, with the tree's base fixed and its 15 m length unchanged.

Towards A: the top rests 12 m up the wall, so the horizontal gap is

\[ \sqrt{15^{2} - 12^{2}} = \sqrt{225 - 144} = \sqrt{81} = 9 \text{ m} \]

Towards B: the top rests 9 m up, so

\[ \sqrt{15^{2} - 9^{2}} = \sqrt{225 - 81} = \sqrt{144} = 12 \text{ m} \]

The tree stands between the two houses, so the distance between them is the sum:

\[ 9 + 12 = 21 \text{ m} \]

Both are 9-12-15 triangles, the 3-4-5 triple scaled by three.

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