After evaluation, The value of the integral is -3.
Calculation:
The triangular region R with vertices (0,0), (2, 1) and (1,2)
And also the transformation is x = 2u+v, y= u+2v
The sketch of the region with vertices A(0,0), B(2,1), and C(1, 2) is as follows:
The straight line passing through P(x₁,y₁) and Q(x₂,y₂) is

The line R₁ passes through points A(0,0) and B(2,1)
So, the equation of the line R₁ is

⇒ 
The line R₂ passes through points B(2,1) and C(1,2).
So, the equation of the line R₂ is

y=3-x
The line R₃ passes through the points C(1,2) and A(0,0): So, the equation of the line R₃ is

y=2x
The transformation of R₁ is x=2u+v,y=u+2v


v=0 ------- (1)
The transformation on R₂ is x=2u+v,y=u+2v
x+y=3
(2u+v)+(u+2v)=3
u+v=1 ---------(2)
The transformation on R₃ is x=2u+v,y=u+2v
y=2x
u+2v=2(2u+v)
u=0 -------(3)
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