Final answer:
The writer varies syntax in the sentence by separating a related idea using parentheses.
Step-by-step explanation:
The writer varies syntax in the sentence by separating a related idea using parentheses. This is evident in the phrase 'though many before him, including children and adults, have claimed to slide seamlessly down a hill while standing on a single flat surface'. By using parentheses, the writer provides additional information without disrupting the flow of the main sentence.
Learn more about syntax variation in writing