posted by alex

the problem is
2cos^2x + sinx-1=0

the directions are to "use an identity to solve each equation on the interval [0,2pi).

This is what i've done so far:



cos2x + sinx =0

1 - 2sin^2x + sinx = 0



I think i'm supposed to factor it next but i'm not sure?


would doing that step be correct?

  1. alex

    although, after looking at this problem i'm still not sure why i had to change the original problem? I've just been following the notes that were given to me by my teacher and that's what she has done but why do i have to change the original problem it seems like just a bunch of extra work

  2. Dena

    replace cos^2x by 1-sin^2x

    2(1-sin^2x) + sinx -1 = 0

    2-2sin^2x +sinx -1 = 0


    2sin^2x - sinx - 1=0

    (2sinx + 1)(sinx -1)=0

    we will get

    sinx = -1/2 and 1

  3. Reiny

    carrying on where Dena left off....

    sinx = -1/2 (angle in standard postition = 30º)
    x must be in quadrant III or IV
    x = 180+30 =210 or
    x = 360-30 = 330

    if sinx = 1
    x = 90

    so in degrees x = 90, 210 or 330

    or in radians
    x = pi/2, 7pi/6 or 11pi/6

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