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So first I did a force balance (dFx=0)
P=PB-PC=8.5kN
RD= RA - P
RA = RD + P
I know the total change in length is equal to 0, so
dAB + dBC + dCD = 0
==>
(RA*L1)/(E*A1) + (P*L2)/(E*A2) + ((RA-P)*L1)/(E*A1) = 0
but when I...