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 #2
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+2

I like doing this in the following manner....construct a square with a side  = 4  [ any side length would work ]

Let A  = (0,0)  B  = (0,4)  C = (4,4)  and D  = (4,0)  E  = (2,4)  F  = (4,2)  M = (0, 2)

 

Here's a pic : 

 

 

Triangles BCF  and DAM are congruent right triangles

 

And the slope of BF  =  [ 4-2] / [ 0 - 4  ]=  2/-4  = -1/2

And the slope of EA =   [ 4-0]  / [2-0]  = 4/2  = 2

So....these segements have reciprocal slopes  so they are perpendicular

 

This means that triangles   BCF  and BGE  are similar by AA congruency

 

BC  = 4     CF  = 2     BE  = 2    BF  =  √ [ BF^2 + CF^2]  = √ [4^2 + 2^2] = √20  = 2√5

 

So.....by similar  triangles......  BG / BE   =  BC/ BF   →  BG/ 2  = 4/ [ 2√5 ]   →  BG  = 4/√5 

 

And GE  =   √[BE^2  - BG^2]  = √[2^2 - (16/5]  = √  4 - (16/5]  = √ [ 20 - 16] / √5  =  2/√5   

 

And the  area of triangle  BCF  =  (1/2) BC * CF  = 4

And since BG / BC  =  (  4 / [ √5] ) / 4   =  1/ √5.....the area  of triangle BGE  = 4 (1/√5)^2  =  4/5

 

Likewise  triangles MAD  and  MNA   are similar  and we can show that triangles BGE  and ANM are congruent

 

So....BG/MN  = BG / GE   =  [4/√5 ] / [2 /√5]    =   2

 

And FG  =  BF - BG   =     2√5  - 4/√5   =  [10 √5 - 4√5] / 5  =  6√5/5  =  6/√5

 

So....FG/ MN  =  6/√5 / [ 2/√5] =  6/2  =  3

 

And DN  = DM - MN  =   BF - GE =  2√5  - 2/√5  =  [ 10√5 - 2√5] / 5 = 8√5/5  = 8/√5

 

So DN / MN  = [8/√5]/ [2/√5]  = 8 /2  =  4 

 

 

Area  of BMDF  =  Area of the square  less areas  of BCF  and DAM  =  4^2  - 2 [4] = 16 - 8  = 8

 

Area of  GBMN  =  area of BEA - areas of BGE  and ANM  =  (1/2)BE * BA  - 2(4/5)  = 

(1/2)(2)(4) - 8/5   =

4- 8/5   =

[20 - 8] / 5  =

12/5

 

So the area of  GFDN  = area of BMDF  - area of GBMN  =   8 - 12/5 =  [40 - 12]/5 = 28/5

 

So  GFDN / GBMN  =  [ 28/5] / [ 12/5] =  28/12   =  7 / 3

 

 

cool cool cool

01.09.2019