In an equilateral triangle, the orthocentre coincides with the centroid.
The centroid divides the altitude from the vertex to the opposite side in the ratio 2:1.
Let M be the foot of the perpendicular from P(3,5) to the line QR given by x+y−4=0. The coordinates of M can be found using the formula:
1x−3=1y−5=−12+123+5−4
1x−3=1y−5=−2
This gives x=1 and y=3. Thus, M is (1,3).
The orthocentre H(α,β) divides the segment PM internally in the ratio 2:1. Using the section formula:
α=2+12(1)+1(3)=35
β=2+12(3)+1(5)=311
Adding α and β:
α+β=35+311=316
Therefore, 9(α+β)=9(316)=48
Answer: 48