Math Is Fun Forum

ninjaman

Member

Registered: 2013-10-15

Posts: 61

trig problem

hello
i have a problem that im stuck on, the question is this

from a ship at sea, the angles of elevation of the top and bottom of a vertical lighthouse standing on the edge of a vertical cliff are 31degrees, and 26 degrees respectively. if the light house is 25m high, how high is the cliff?

so far i have h+25/tan31 = h/tan26

rearrange to, (h+25)tan26 = h tan31

im lost after this

any help would be appreciated.

thanks
simon:)

Bob

Administrator

Registered: 2010-06-20

Posts: 10,629

Re: trig problem

hi Simon,

You've done the correct thing so far and I got this, too. 

So multiply the bracket:

Move the h terms together:

Factorise the h:

And divide by the bracket ........

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

ninjaman

Member

Registered: 2013-10-15

Posts: 61

Re: trig problem

hello bob

thanks for the help!

I divided by the brackets? (I think), meaning I did what was in the brackets first

(tan31-tan26) = 0.1131280305 * h = 25 tan 26

then, divided the first half by 0.1131280305 to move it over and under to get

h = 25 tan 26
      0.1131280305 = 107.78332

is this correct, I have a feeling you meant something else by divide by the brackets and now im way off.

cheers
all the best
simon

Bob

Administrator

Registered: 2010-06-20

Posts: 10,629

Re: trig problem

hello Simon,

That's spot on; well done! 

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

ninjaman

Member

Registered: 2013-10-15

Posts: 61

Re: trig problem

that's great!

thanks for your help bob!

all the best

simon:)