Strain gauge - determination of x and y strains

1. The problem statement, all variables and given/known data

Determine x and y strains in order to construct a Mohr's circle of strain.

A 2-gauge Wheatstone (the third/middle gauge was not working), with gauges 90-degrees apart, was attached to a specimen steel plate 15-degrees offset from the x and y axes. An axial load was applied to the plate on an Olsen tensile testing machine.

2. Relevant equations

Strain 1 = ((strain-x + strain-y)/2) + ((strain-x - strain-y)/2) cos (2theta) + ((shear strain-xy)/2) sin (2theta) = -65 micro strain

Strain 2 = ((strain-x + strain-y)/2) + ((strain-x - strain-y)/2) cos (2theta) + ((shear strain-xy)/2) sin (2theta) = 278 micro strain

These equations are usually used when a three-gauge rosette is used. The lecturer who set this assignment has moved to another university, and I am unable to reach him at the moment. The lecturer who took his place told me, over the telephone, that these equations only apply to a 3-gauge rosette and cannot be used for this problem. The new lecturer uses all three gauges for his experiment.

There is nothing in my notes explaining how to determine x and y strains where only two gauges of a rosette were used. Please could you help me to work out the x and y strains?
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 Recognitions: Homework Help Science Advisor Presumably, E = 200 GPa, and nu = 0.30. Hint: epsx = (sigmax - nu*sigmay)/E, and epsy = (sigmay - nu*sigmax)/E. But by inspection, what is the value of sigmay? Now substitute into your given relevant equations, and solve for the unknowns.
 Thanks for your reply. Unfortunately, one of the aims of this assignment is to use the strain gauge data in order to determine Poisson's and Young's. I am unable to use them to calculate for strain.

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