Bonus question: if we know the orientation of the strain ellipse axes (the y-axis of our new coordinate frame), then for two differently oriented line elements (orientations ea and (b) with reciprocal quadratic extensions i'a and i'o, we can formulate two equations to describe how these features are related to the two principal reciprocal quadratic extensions a'i and x2: l'a = x 1cos20'a + 'zsine'a a'p = 'icos20'+ a'zsine's use these two equations with two unknowns to solve for 21 and 22 and their orientations.