I need more information. Find the midpoint of what? Do you mean find the centroid of the triangle? Assuming that line AB is horizontal and P is the midpoint of AB, then the centroid is on the line OP.
One very nice method of finding the centroid of a triangle (doesn't work for figures of more than three sides) is to "average" the three vertices. That is, if the vertices of this triangle are A(-a, b), B(a, b), C(0,0) as it appears, then the centroid is a ((-a+a+ 0)/3, ((b+b+0)/3)= (0, (2/3)b).
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