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Since the negative moment is acting on the section whose area vector is negative, bending moment is positive.This is the equation of straight line. We can easily drawn the bending moment diagram, by finding out the bending moment at x = 0 and x = L and joining the points at x = 0 x = L, by straight line. At x = L, M2 = 0 and x = 0 M2 = - PL .
Beams

For finding out the bending moment, consider the moment balance of part 2, taking the moment about C

M 2 + P(L-x) = 0

M 2 = -P (L-x)