What is the similarity between a red blood cell and a root hair cell?

Both cells have a high surface area to volume ratio.

RBC's lose their nucleus before going into the bloodstream. The job of both red blood cells and root hair cells is absorption. Red blood cells absorb oxygen through pulmonary veins and their capillaries in the lungs. Root hair cells, on the other hand, absorb water from the soil and transport them to adjoining cells for photosynthesis purposes.

Since both's function involves transportation of essential substances, evolution has maximized their usage potential. They have a large surface area for maximum absorption of nutrients and hence this ratio is high.