Show that the area of triangle DEF is 11 cm2.​

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Explanation:

The length CD can be found from the Pythagorean theorem.

  CF² + CD² = DF²

  2² + CD² = (2√10)²

  CD² = 40 -4 = 36

  CD = √36 = 6

From here, any of several methods could be used to find the area of ΔDEF.

One of them is to subtract the areas of the triangles that are not ΔDEF from the area of the rectangle ABCD.

  area ABCD = bh = (6 cm)(5 cm) = 30 cm²

  area ΔADE = 1/2bh = 1/2(4 cm)(5 cm) = 10 cm²

  area ΔBEF = 1/2bh = 1/2(2 cm)(3 cm) = 3 cm²

  area ΔCDF = 1/2bh = 1/2(6 cm)(2 cm) = 6 cm²

Then the area of interest is ...

  area ΔDEF = area ABCD -area ΔADE -area ΔBEF -area ΔCDF

  area ΔDEF = (30 -10 -3 -6) cm²

  area ΔDEF = 11 cm²

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Alternate solution

If we define A as the origin of a coordinate system, then the coordinates of the vertices of ΔDEF are ...

  D(0, 5), E(4, 0), F(6, 3)

The area can be computed as half the absolute value of the sum of the determinants of 2×2 matrices consisting of adjacent points.

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Additional comment

The solution using coordinates is effectively equivalent to the computation of the area of trapezoid ABFD, less the areas of triangles EBF and AED.