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Given : AB || DE and BC || EF. Prove that :

(i) ADDG=CFFG\dfrac{AD}{DG} = \dfrac{CF}{FG}

(ii) △DFG ~ △ACG.

Given : AB || DE and BC || EF. Prove that AD/DG = CF/FG. Similarity, Concise Mathematics Solutions ICSE Class 10.

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Answer

(i) In △ABG, DE || AB.

So, by basic proportionality theorem we get,

DGAD=GEBE\dfrac{DG}{AD} = \dfrac{GE}{BE} …..(1)

In △BCG, EF || BC.

So, by basic proportionality theorem we get,

GEBE=FGCF\dfrac{GE}{BE} = \dfrac{FG}{CF} …..(2)

From (1) and (2) we get,

DGAD=FGCFADDG=CFFG.\Rightarrow \dfrac{DG}{AD} = \dfrac{FG}{CF} \\[1em] \Rightarrow \dfrac{AD}{DG} = \dfrac{CF}{FG}.

Hence, proved that ADDG=CFFG.\dfrac{AD}{DG} = \dfrac{CF}{FG}.

(ii) In △DFG and △ACG,

ADDG=CFFG\dfrac{AD}{DG} = \dfrac{CF}{FG} [Proved above]

⇒ ∠DGF = ∠AGC [Common angle]

∴ △DFG ~ △ACG [By SAS]

Hence, proved that △DFG ~ △ACG.

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