Answer:
The answer to your question would be that the a combination of the sentences above would be as follows: I ran three miles, so I was tired.
Step-by-step explanation:
The sentence above is a complex sentence: It contains a independent clause and a dependent one. The first one represents a complete thought and can stand alone as a sentence, whereas the second one fails to do so (see A). Subordinating conjunctions link a dependent clause to a independent one. The subordinating conjunction indicates that the dependent clause has informative value to add to the sentence's main idea.
a) *So I was tired
A shows that the clause introduced by a subordinating conjunction cannot stand alone as a sentence because it does not represent a complete thought. It depends on the main clause to complete its meaning.