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mathslover

  • 2 years ago

Basic trigonometry help needed.

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  1. mathslover
    • 2 years ago
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    Prove : \(\sin (90 - \theta) = \cos \theta\)

  2. mathslover
    • 2 years ago
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    Actually I am confused that how can I : prove that sin in 2nd quadrant is +ve ? That is my basic confusion ...

  3. hartnn
    • 2 years ago
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    can u use sin (A-B) formula?

  4. mathslover
    • 2 years ago
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    No idea. Actually that was just in start and the formula arises later, so I think that there must be an easier method?

  5. hartnn
    • 2 years ago
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    draw a right triangle

  6. mathslover
    • 2 years ago
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    :( not so good but OK ? :)

  7. mathslover
    • 2 years ago
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    |dw:1350046739720:dw| nothing is there without naming , OK now I think.

  8. hartnn
    • 2 years ago
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    |dw:1350046691525:dw| now find sin 90-theta and cos theta using basic defination

  9. mathslover
    • 2 years ago
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    OK wait please !

  10. mathslover
    • 2 years ago
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    In case of sin 90 - theta , will perpendicular be : adjacent (base) and base will be opposite?

  11. hartnn
    • 2 years ago
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    know the basic defination of sin and cos ? |dw:1350046897401:dw| apply it here, in terms of a and b,c

  12. mathslover
    • 2 years ago
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    OH IT IS soh cah toa and not soa cah toa , sorry :(

  13. mathslover
    • 2 years ago
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    sin = opposite / hypotenuse cos = adjacent / hypotenuse

  14. hartnn
    • 2 years ago
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    yup

  15. mathslover
    • 2 years ago
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    sin (90-theta) = b/c cos (theta) = b/c and hence sin(90-theta) = cos(theta) , got it now? lol jk. Well is this right @hartnn

  16. hartnn
    • 2 years ago
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    yep, hence proved.

  17. hartnn
    • 2 years ago
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    next identity please.

  18. mathslover
    • 2 years ago
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    prove that sin in 2nd quadrant is +ve ? That is my basic confusion ...

  19. hartnn
    • 2 years ago
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    here, put theta = -theta

  20. hartnn
    • 2 years ago
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    and use cos(-theta) = cos theta

  21. mathslover
    • 2 years ago
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    how to prove that cos(-theta)=theta ?

  22. mathslover
    • 2 years ago
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    again with the use of right triangle @hartnn ?

  23. hartnn
    • 2 years ago
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    how will u find -theta in triangle ?

  24. mathslover
    • 2 years ago
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    oh yes .. silly me . So, a cartesian plane ?

  25. mathslover
    • 2 years ago
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    no, cartesian plane will not work also.

  26. sara12345
    • 2 years ago
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    ccw : +ve angle cw : -ve angle

  27. hartnn
    • 2 years ago
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    because lengths will be positive , so ratio will always come out to be positive, hence, maybe cartesian plane won't work

  28. mathslover
    • 2 years ago
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    Some how agreed .

  29. mathslover
    • 2 years ago
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    searched net now, http://www.mathsisfun.com/algebra/trig-four-quadrants.html is that the correct one ?

  30. hartnn
    • 2 years ago
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    yes, its correct....we could prove it using co-ordinates.... (cos t,sin t)= (x,y) so sin t is +ve in 2nd quadrant

  31. mathslover
    • 2 years ago
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    doubts ended :) thanks a lo.

  32. mathslover
    • 2 years ago
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    *ton/lot.

  33. hartnn
    • 2 years ago
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    mathisfun is always a nice reference....

  34. sara12345
    • 2 years ago
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    more we knw more confuse we become

  35. hartnn
    • 2 years ago
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    more we practice, more the confusion vanishes....

  36. sara12345
    • 2 years ago
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    more we practice, more ways we look at things n hence more confusions which ofcourse gives kick ;p

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