A triangle has a vertex at (1, 2) and the mid points of the two sides through it are (-1, 1) and (2, 3). Then the centroid of this triangle is 

Let A(1, 2), B, Cbe the vertices of the triangle 
D(-1, 1), E(2, 3) be the midpoints of AB, AC respectively. 
B= 2D-A =2(-1, 1)-(1,2) =(-3, 0) 
C= 2E-A = 2(2, 3) -(1, 2) = (3, 4)
Centroid of $△$ABC is $\left(\frac{1-3+3}{3},\frac{2+0+4}{3}\right)=\left(\frac{1}{3},2\right)$