A level maths help trig

Help I don’t understand this part of the question why do I multiply 3600 by 2 to get the7200 part on the first line of markscheme and how do I know that I multiply the amount of roots for 0<_x<360 by 20???

(edited 1 year ago)

Reply 1

tonyiptony

Maybe a simplified question will help you.

We know that cos(x)=0 has two roots from 0<=x<360 degrees (being 90 degrees and 270 degrees). Now...
(i) How many roots does cos(2x)=0 have for 0<=x<360? How about cos(3x)=0 Can you generalize some idea for cos(nx)?
(ii) How many roots does cos(x)=0 from 0<=x<720? How about 0<=x<360*3? Can you generalize something?

P.S. I tend not to think of these as formulas, as I don't "deduce solutions" that way. But I know (i) some people do have this memorized; and (ii) the question asks for it, so... let's try.