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 #4
avatar+130511 
+2

Using the Law of Sines   ih the lower left  blue triangle, we have

 

sin  (a)  / s  =  sin (60) / S       (1) 

 

Where  a  is the angle formed by the side of the yellow triangle  and the base of this triangle

And S is the  side of the  yellow triangle

And s  is the  side of the orange triangle with an area of 20

 

And using the Law of Sines  in  the lower right blue triangle, we have

 

sin ( 120 - a)  / (1.5s)  =  sin (60) / S     (2)

 

Where  (180 - 60 - a)  =   (120 - a)   is the angle formed  by  the side of the  yellow triangle  and the  base of this triangle

And S is the side of the yellow triangle

And 1.5s  is the side of the red triangle

 

So....equating (1) and (2), we have

 

sin (a) / s   =    sin (120 - a) / (1.5 s)

 

sin (a)    =   sin (120 - a) / (1.5)

 

sin (a)  =  [sin(120) cos(a)  -  sin (a)cos (120) ]  / (3/2)

 

(3/2)sin (a)   = (√3 / 2 )cos (a)  +   sin (a) (1/2)

 

3sin (a)  = √3 cos (a)  +  sin a

 

2sin (a)  =  √3 cos (a)

 

sin (a) / cos(a)  =  √3/2

 

tan (a)  = √3/2

 

So  sin (a)   =    √ 3  / [ √ [ 3 + 4 ]  =  √3 / √7

 

And we  can find s   as

 

20  = (√3/4)s^2

 

80 / √3  = s^2

 

s  = √ [ 80 / √3  ] 

 

And we can find the side  of the yellow triangle as 

 

s / sin (a)  = S  / sin (60)

 

√ [ 80 / √3  ]  / [ √3 / √7 ] =   S / [√3/2 ]

 

S =  (√3/2) * √ [ 80 / √3  ]  / [ √3 / √7 ]  =      2√35 / [4√3 ]

 

So....the area  of the  yellow equilateral  triangle is

 

(√3 / 4)S^2  =

 

(√3/ 4)  *  (2√35)^2 / √3  =

 

(1/4) 140   =

 

140 / 4  =

 

35  units^2

 

 

cool cool cool

Jan 3, 2020
Jan 2, 2020
 #5
avatar+17 
+1
Jan 2, 2020

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