I really can't graspthe concept of math

[Sir_Laughalot]

Here's another question I can't do and it's similar to one I asked earlier on this forum.

But I just can't get the idea of how to solve it.

Can you people sort of show me how you got the answer with the solution as well....thanks for your help.

The minute and the hour hands of a clock are exactly at right angles. How long will it take for this sitation to occur again?

Stupid math !

[Dnereb]

How about take a clock or watch with hands and mov it forward to Three o clock. Then move it forward again until just a bit over half past four
look at the hands and think about this....
Go forward until yuo reach 5 minutes past four.
A right angle again only at the two next numbers on the clock.

So in every hour and 5 minutes this sitiuation occurs twice.
ergo in 32.5 minutes it occurs again!

And I didn't use a clock for this... it's deduction and imagination you need not math in this case

[Sir_Laughalot]

Yeah....too bad you need math to get full marks for the question....


Thanks for your help...... : rolls eyes : .

[Dnereb]

So it's homework you need to be answered.... I won't give you the formula
but some clues

Okay think about these....

The large hand rotates once in 24 hours
and the little hand rotates 24 times in 24 hours
their ratio is 1:12
So the angle of the little hand is 12 times the angle of the large hand
Define the angle for each hand
whith which what sinus diffrences wil the angle be 90 degrees (exactly)
in simple words
if Abs(sin(angle large hand) - Sin(Angle little hand )) = 1 then Hands at 90 degree angle

Now you should be able to work it out from here or you shouldn't pas at all

[Dnereb]

Sorry
this one is wrong, didn't think about the signs propper:

if Abs(sin(angle large hand) - Sin(Angle little hand )) = 1 then Hands at 90 degree angle

Make it:
if Abs(sin(angle large hand)) * -1 - Abs(Sin(Angle little hand )) = -1 then Hands at 90 degree angle

Edit:
or rather...
if Abs(sin(angle large hand)) + Abs(Sin(Angle little hand )) = 1 then

[Sir_Laughalot]

To be frank.......Although I'm in 10th grade at my school...I haven't learnt about Cos, sin or the rest .

But I'll look into them.

[Logophobic]

Go forward until yuo reach 5 minutes past four.
A right angle again only at the two next numbers on the clock.

At 4:05, the hands of the clock are not at a right angle. This should be obvious, because the hour hand does not stay on the 4 while the minute hand moves from 12 to 1. The solution is not as straightforward as Dnereb suggest, but this problem can still be solved without sine or cosine.

Consider the times when you know the hands are at a right angle; i.e. 3:00 and 9:00. During this 6-hour period, find the times when the hands are at approx. right angle. These times are:

3:35
4:05
4:40
5:10
5:45
6:15
6:50
7:20
7:55
8:25

As you can see, there are 10 times between 3:00 and 9:00 when the hands are at right angle. This divides the 6-hour period into 11 equal time slices. Simple division will give you the exact length of these. Six hours = 360 minutes; divide by 11 and you get 32.72727... minutes.

[Dnereb]

To be frank.......Although I'm in 10th grade at my school...I haven't learnt about Cos, sin or the rest .

But I'll look into them.

You can leave them out if you want to :
if angle large hand Mod 90 = Angle little hand Mod 90 then

and if you transpose the first clue it says:
The angle of the little hand = (angle of the large hand * 12) Mod 360
So you can subtitute on for the other

Glad to be of no help

Edit adjusted the formula without the sinus...again

[Harsh Gupta]

The large hand rotates once in 24 hours
and the little hand rotates 24 times in 24 hours

i think you want to say that:

the hour (or small) hand rotates twice in 24 hours, and
the minute (or large) hand rotates 24 times in 24 hours!!

[Dnereb]

Oeps!

[Logophobic]

You guys must have very strange clocks. The hour hand on my clock rotates twice in 24 hours...

[Harsh Gupta]

yes you are right!! "I really can't graspthe concept of math"