Geometry

Question Analysis  
This question involves using a tree diagram to calculate the probability of a compound event (at least one head) by identifying all valid paths: head-head, head-tail, tail-head.
The main focus is on using the addition rule for multiple successful paths in independent events.  

Key Concept Explanation  
"At least one head" includes all outcomes except tail-tail.
In a tree diagram, each flip is independent, so path probabilities are multiplied, and valid paths are summed.  

Step-by-Step Solution  
1. Path 1: Head-Head  
Probability:  
2. Path 2: Head-Tail  
Probability:  
3. Path 3: Tail-Head  
Probability: