M1 - moments in equilibrium

I just wanted to clarify if the solution is wrong or not.

The question:

A 9 m long non-uniform beam, AB, of weight 240 N, is suspended between two trees to make a walkway. It is held in equilibrium by vertical wires at C and D. The magnitude of the tension in the wire at C is twice the magnitude of the tension in the wire at D.

I solved part a) but part b) is:

Find the distance of the centre of mass of the beam from A.

This is what the diagram looks like:


I did as what the solution showed, but I had a 7 instead of the 8, because the beam is 9 m long and the mid section of the diagram would be 7? To make a total of 9, by adding the 1 m near A and the 1 m near B.

The solution is taking moments about A. D is 8 m from A.

Where were you taking moments about? If you take moments about C, there won't be a term involving Tc, the term involving Td will be Td x 7 and the distance you work out won't be from A so you'll have to add 1.

I did do it from A. I realised my mistake once you said: "D is 8m from A". Sorry! What a silly mistake...