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Question

D, E, and F are the mid-points of the sides BC, CA and AB, respectively of a ΔABC. Then find the ratio of the areas of ΔDEF and ΔABC.

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Solution



D and E are the mid-points of the sides BC and AB respectively of ΔABC.

DEBA, DE=12×BA [ Mid-point theorem]

DEFA(1)

Since, D and F are mid-points of the sides BC and AB respectively of ΔABC,

DFCADFAE....(2)

From (1) and (2) we conclude that AFDE is a parallelogram

Similarly, BDEF is a parallelogram

Now, in ΔDEF and ΔABC, we have

FDE=A [Opposite angles of parallelogram AFDE]

and DEF=B [Opposite angles of parallelogram BDEF]

ΔDEFΔABC [AA-similarity criterion]

Area (ΔDEF)Area (ΔABC)=DE2AB2

Area (ΔDEF)Area (ΔABC)=(12AB)2AB2=14

[DE=12×AB]

Hence, Area (ΔDEF):Area (ΔABC)=1:4


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