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Constructions

Question
CBSEENMA9002908

In figure, D and E are two points on BC such that BD = DE = EC. Show that ar(ΔABD) = ar(ΔADE) = ar(ΔAEC). [CBSE 2012 (March)]

Can you now answer the question that you have left in the ‘Introduction’, of this chapter, whether the field of Budhia has been actually divided into three parts of equal area?



[Remark: Note that by taking BD = DE = EC, the triangle ABC is divided into three triangles ABD, ADE and AEC of equal areas. In the same way, by dividing BC into n equal parts and joining the points of division so obtained to the opposite vertex of BC, you can divide ΔABC into n triangles of equal areas.] 

Solution
Let AM ⊥ BC. Then, the height of each of ΔABD, ΔADE and ΔAEC is the same.

therefore space ar left parenthesis increment ABD right parenthesis equals 1 half cross times BD cross times AM space space space space space space space space space space space space space space space space... left parenthesis 1 right parenthesis
space space space space space space space space ar space left parenthesis increment ADE right parenthesis equals 1 half cross times DE cross times AM space space space space space space space space space space space space... left parenthesis 2 right parenthesis
and comma space space ar space left parenthesis increment AEC right parenthesis equals 1 half cross times EC cross times AM space space space space space space space space space space space... left parenthesis 3 right parenthesis

∵ BD = DE = EC ∴ From (1), (2) and (3), ar (ΔABD) = ar (ΔADE) = ar (ΔAEC)
Yes, the field of Budhia has been actually divided into three parts of equal area.