Final answer:
Without access to the specific textbook, it is uncertain which equations (2.4 or 2.9) can be used to express stress in terms of strain components. Generally, Hooke's Law, which relates stress linearly with strain, can be rearranged to achieve this expression.
Step-by-step explanation:
When inverting Hooke's Law to express stress in terms of strain components, it is unclear which specific equations Equation (2.4) and Equation (2.9) refer to without the context of the textbook or source material you are using. However, Hooke's Law typically linearly relates stress and strain, in the elastic regime, with the stress being the product of the modulus of elasticity (often denoted as Y or E) and the strain. This relation can be expressed as stress = Y × strain. If the material behaves linearly and elastically, then this equation can be rearranged to solve for stress as a function of strain, or vice versa. It's important to note that both tensile and compressive stress and strain can be described by similar formulas, with the difference that in compression, absolute values are considered.