(Requires Appendix material)A necessary and sufficient condition to derive the OLS estimator is that the following two conditions hold: 
= 0 and 
= 0.Show that these conditions imply that 
= 0.
Definitions:
Sandstone
A sedimentary rock consisting of sand-sized minerals or rock grains, compacted and often cemented together over time.
Limestone
A sedimentary rock composed primarily of calcium carbonate, often formed from marine organisms' shells and skeletons.
Granite
A rough-textured igneous rock that predominantly contains quartz, feldspar, and mica, frequently used in the construction industry.
Lineation
A linear feature within rock, such as grain elongation or mineral stretching, that indicates flow or movement directions during formation.
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