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
