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Question 8
In the following figure, ABC is a right triangle right angled at A. BCED, ACFG and ABMN are squares on the sides BC, CA and AB respectively. Line segment AXDE meets BC at Y. Show that:

(i) ΔMBCΔABD
(ii) area(BYXD)=2area(ΔMBC)
(iii) ar(BYXD)=ar(ABMN)
(iv) ΔFCBΔACE
(v) ar(CYXE)=2ar(ΔFCB)
(vi) ar(ACFG)=ar(CYXE)
(vii) ar (BCED) = ar(ABMN) + ar(ACFG)

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Solution

(i) We know that each angle of a square is 90.
Hence, ABM=DBC=90
ABM+ABC=DBC+ABC
MBC=ABD
In ΔMBC and ΔABD,
MBC=ABD (Proved above)
MB = AB (Sides of square ABMN)
BC = BD (Sides of square BCED)
ΔMBCΔABD (SAS congruence rule)
(ii) We have, ΔMBCΔABD
ar(ΔMBC)=ar(ΔABD)...(1)
It is given that, AXDE and BDDE. (Adjacent sides of square BDEC)
BD||AX (Two lines perpendicular to same line are parallel to each other)
ΔABD and parallelogram BYXD are on the same base BD and between the same parallels BD and AX.
ar(ΔABD)=12ar(BYXD)
i.e., ar(BYXD)=2ar(ΔABD)
area(BYXD)=2area(ΔMBC) [Using equation (1)] ... (2)
(iii) ΔMBC and parallelogram ABMN are lying on the same base MB and between same parallels MB and NC.
ar(ΔMBC)=12ar(ABMN)
i.e., 2ar(ΔMBC)=ar(ABMN)
ar(BYXD)=ar(ABMN) [Using equation (2)] ... (3)
(iv) We know that each angle of a square is 90.
FCA=BCE=90
Then, FCA+ACB=BCE+ACB [Adding ACB on both sides]
FCB=ACE
In ΔFCB and ΔACE,
FCB=ACE [proved above]
FC = AC (Sides of square ACFG)
CB = CE (Sides of square BCED)
ΔFCBΔACE (SAS congruence rule)
(v) It is given that, AXDE and CEDE. (Adjacent sides of square BDEC)
Hence, CE || AX (Two lines perpendicular to the same line are parallel to each other)
Consider ΔACE and parallelogram CYXE.
ΔACE and parallelogram CYXE are on the same base CE and between the same parallels CE and AX.
ar(ΔACE)=12ar(CYXE)
i.e., ar(CYXE)=2ar(ΔACE)...(4)
We had proved that,
ΔFCBΔACE
ar(ΔFCB)=ar(ΔACE)...(5)
On comparing equations (4) and (5), we obtain,
ar(CYXE)=2ar(ΔFCB)...(6)
(vi) Consider ΔFCB and parallelogram ACFG.
ΔFCB and parallelogram ACFG are lying on the same base CF and between the same parallel CF and BG.
ar(ΔFCB)=12ar(ACFG)
i,ear(ACFG)=2ar(ΔFCB)
ar(ACFG)=ar(CYXE) [Using equation(6)]... (7)
(vii) From the figure, it is evident that,
ar (BCED) = ar(BYXD) + ar(CYXE)
ar (BCED) = ar(ABMN) + ar(ACFG) [Using equations (3) and (7)]


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