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Ireland geometry
Problem
Prove that a line through the centroid of a triangle that bisects the area of the triangle is a median.
Solution
Let , with on and on , bisect the area of . Let the midpoints of and be and respectively and let the centroid of be . Assume passes through and is not a median, i.e. and .
Since bisects the area of then the area of is equal to the area of , hence the area of is equal to the area of . This means that hence the last equality because of . This implies . Because we get and so hence . This is impossible so cannot bisect the area of unless it is a median.
Since bisects the area of then the area of is equal to the area of , hence the area of is equal to the area of . This means that hence the last equality because of . This implies . Because we get and so hence . This is impossible so cannot bisect the area of unless it is a median.
Techniques
Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleTriangle trigonometryAngle chasing