Let n ≥ 3. How many n-symbol strings over this alphabet contain exactly 3 “words”? (Hint: use Example 9.4 to account for n-symbol strings with exactly two ␣s; then use Inclusion–Exclusion to prevent...

Let n ≥ 3. How many n-symbol strings over this alphabet contain exactly 3 “words”? (Hint: use Example 9.4 to account for n-symbol strings with exactly two ␣s; then use Inclusion–Exclusion to prevent initial/final/consecutive spaces, as in ␣ABC· · · , · · · XYZ␣, and · · · JKL␣␣MNO· · · .)