expand ( 5d-1) 4

Answer 1

Do you mean (5d -1)^4 ?

Square 5d -1 twice.

The first squaring will give you
25d^2 -10d +1

Now square that. There will be more terms next time.

This is not trigonometry

Answer 2

Some people call the nth harmonic the nth ABOVE the fundamental and do not call the fundamental a harmonic. In that case, your answer would be 104/8 = 13 cm

Answer 3

Whoops. Ignore last answer. I posted it in the wrong place