creative8401

Im Anush Hayastan
10+ Year Member
Jan 22, 2008
202
2
CA
Status
Pre-Dental
Hi everybody. Anyone with good trig skills, will you please help me on this problem. It's either so simple i cant see it, or something wrong with their answer:

Problem:
Cos ([Pi]+[301Pi/2])

Their Explanation:
Cos ([Pi]+[75*2Pi]+[Pi/2]) = Cos ([Pi]+[Pi/2]) = 0

I don't know what happened to [75*2Pi], and Cos([Pi]) = -1 & Cos ([Pi/2]) = 0, so -1+0 doesn't give me 0 either. If you could shed some light, much appreciated.
 

doc3232

10+ Year Member
7+ Year Member
Feb 15, 2008
3,809
9
Status
Dental Student
Hi everybody. Anyone with good trig skills, will you please help me on this problem. It's either so simple i cant see it, or something wrong with their answer:

Problem:
Cos ([Pi]+[301Pi/2])

Their Explanation:
Cos ([Pi]+[75*2Pi]+[Pi/2]) = Cos ([Pi]+[Pi/2]) = 0

I don't know what happened to [75*2Pi], and Cos([Pi]) = -1 & Cos ([Pi/2]) = 0, so -1+0 doesn't give me 0 either. If you could shed some light, much appreciated.
ok
so
cos (2 Pi) is a full loop
so cos (150Pi) is 75 loops and also has 0 effect on the answer, whats left?
cos ( pi + 301Pi/2)= cos(150Pi+3Pi/2) as I said 75 loops has no influence on answer so all that is left is cos(1.5Pi) which is equal to 0
I know I did not answer 100% clearly but I hope you get it.
cos 2Pi is 0
so is cos 150Pi as is any cos (nPi) where n is divisible by 2.
 

eatkabab

10+ Year Member
Apr 20, 2008
81
1
Status
Dental Student
doc3232 explained it correctly:

cos(Pi + 301Pi/2) is equivalent to cos(150Pi + 3Pi/2)

since 2Pi is just 360 degrees, it doesn't affect the answer, therefore:
150Pi/2Pi = 75.
that is 75 "loops" or, 75 three sixty degree spins. so now you can drop that term completely out of the problem, and you have:

cos(3Pi/2)
which is just 1.5Pi, which is equivalent to 270 degrees, or, pointing straight down south. and when you're pointing down south, cos = 0.



but I got another problem for you.

Sec^2 (A) + Tan^2 (A-3) = 0

apparently the answer is (pi/4) but I'm not 100% sure of that cuz it don't make any sense!!! I've been lookin at this problem and trying to figure it out for a month now. still can't get it...


I just attempted it again and got stuck here:

1 = tan^2 (A-(Pi/60)) - tan^2 (A)
1 = tan (A) [tan (1-(Pi/60A)) - tan (Pi/180)]

I don't know if anything I'm doing is correct by the way. I'm not even sure if I can convert that "A-3" into radians from the beginning...
 

nt4reall

New Member
10+ Year Member
5+ Year Member
Apr 14, 2006
87
0
Status
Pre-Dental
Hi everybody. Anyone with good trig skills, will you please help me on this problem. It's either so simple i cant see it, or something wrong with their answer:

Problem:
Cos ([Pi]+[301Pi/2])

Their Explanation:
Cos ([Pi]+[75*2Pi]+[Pi/2]) = Cos ([Pi]+[Pi/2]) = 0

I don't know what happened to [75*2Pi], and Cos([Pi]) = -1 & Cos ([Pi/2]) = 0, so -1+0 doesn't give me 0 either. If you could shed some light, much appreciated.
Cos (п + 301 п/2 ) = cos (п + ( 300 п/2 + п/2))
= cos (п + ( 150 п + п/2))
= cos (п + п/2) ( we can ignore 150 п)
= cos ( 3 п/2)
If you draw a circle , cos ( 3 п/2) is ¾ of the circle , and then cos ( 3 п/2 = 0 .
Hope this help.