Quantcast

Got Homework?

Connect with other students for help. It's a free community.

  • across
    MIT Grad Student
    Online now
  • laura*
    Helped 1,000 students
    Online now
  • Hero
    College Math Guru
    Online now

Here's the question you clicked on:

55 members online
  • 0 replying
  • 0 viewing

greeneyes<3 Group Title

Evaluate sin (-840) without using a calulator. My answer is (-1/2) but i'm not too good with these so can someone please check it? Thanks:)

  • 2 years ago
  • 2 years ago

  • This Question is Closed
  1. jim_thompson5910 Group Title
    Best Response
    You've already chosen the best response.
    Medals 0

    sin(-840) = -sin(840) .... since sine is an odd function sin(-840) = -sin(480) .... 480 and 840 are coterminal angles sin(-840) = -sin(120) ... 480 and 120 are coterminal angles sin(-840) = -sqrt(3)/2 ... use the unit circle (link given below) to find this value So the answer is sin(-840) = -sqrt(3)/2 Unit Circle: http://www.math.ucsd.edu/~jarmel/math4c/Unit_Circle_Angles.png

    • 2 years ago
  2. greeneyes<3 Group Title
    Best Response
    You've already chosen the best response.
    Medals 0

    Thank you! I figured it out my mistake and then you posted this! I have memorize all the unit circle... :( Thank you so much anyways! Is the answer to sec (-15 pi/4) = sqrt 2/2 ?

    • 2 years ago
  3. jim_thompson5910 Group Title
    Best Response
    You've already chosen the best response.
    Medals 0

    sec(-15pi/4) = 1/cos(-15pi/4) sec(-15pi/4) = 1/cos(15pi/4) ... Note: cosine is an even function sec(-15pi/4) = 1/cos(-pi/4) ... 15pi/4 is coterminal with -pi/4 sec(-15pi/4) = 1/cos(pi/4) sec(-15pi/4) = 1/(sqrt(2)/2) ... Use the unit circle here sec(-15pi/4) = 1*(2/sqrt(2)) sec(-15pi/4) = 2/sqrt(2) sec(-15pi/4) = (2/sqrt(2))*(sqrt(2)/sqrt(2)) ... Rationalizing the denominator sec(-15pi/4) = (2*sqrt(2))/(sqrt(2)*sqrt(2)) sec(-15pi/4) = (2*sqrt(2))/(sqrt(2*2)) sec(-15pi/4) = (2*sqrt(2))/(sqrt(4)) sec(-15pi/4) = (2*sqrt(2))/(2) sec(-15pi/4) = sqrt(2) So the answer is sec(-15pi/4) = sqrt(2)

    • 2 years ago
  4. phi Group Title
    Best Response
    You've already chosen the best response.
    Medals 0

    Thank you so much anyways! Is the answer to sec (-15 pi/4) = sqrt 2/2 ? You found the cos(-15 pi/4) . Do you know how you went wrong?

    • 2 years ago
  5. phi Group Title
    Best Response
    You've already chosen the best response.
    Medals 0

    btw, for sin(-840), I would keep adding 360 until I get to a number I like -840+ 2*360= -120 or -120+360= 240 of course -120 and 240 are the same angle.

    • 2 years ago
    • Attachments:

See more questions >>>

Your question is ready. Sign up for free to start getting answers.

spraguer (Moderator)
5 → View Detailed Profile

is replying to Can someone tell me what button the professor is hitting...

23

  • Teamwork 19 Teammate
  • Problem Solving 19 Hero
  • You have blocked this person.
  • ✔ You're a fan Checking fan status...

Thanks for being so helpful in mathematics. If you are getting quality help, make sure you spread the word about OpenStudy.

This is the testimonial you wrote.
You haven't written a testimonial for Owlfred.