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In △ABC, point D∈ AC with AD:DC=4:3, point E∈ BC so that BE:EC=1:5. If ACDE=5 in2, find ABDC, AABD, and AABC.

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In triangles with the same altitude, area is proportional to the base length. Triangles CDE and BDE both have the same altitude (the perpendicular distance from D to BC), so their areas are in the proportion BE:EC = 1:5. Since ACDE = 5 in², ABDE = 1 in². The area of BDC is the total of those:

... ABDC = 6 in²

Likewise, triangles ABD and CBD both have an altitude that is the perpendicular distance from B to AC. Then, ...

... AABD : ACBD = AD : CD = 4:3

Thus AABD = 4/3 × ACBD = 4/3 × 6 in²

... AABD = 8 in²

Of course, AABC is the sum of AABD and ABDC, so is ...

... AABC = 6 in² + 8 in²

... AABC = 14 in²

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