Bantu dijawab. Jawaban terbaik akan saya jadikan sebagai jawaban tercerdas.

a. cosxcos20° - sinxsin20° = 1/2
    cos(x + 20°) = 1/2
    cos(x + 20°) = cos60°
    x + 20° = 60° + k . 360°
    x = 40° + k . 360°
    x = 40°
    x + 20° = -60° + k . 360°
    x = -80° + k . 360°
    x = 280°
    HP = {40°, 280°}

b. sin100°cos2x - cos100°sin2x = -1/2
    sin(100° - 2x) = -1/2
    sin(100° - 2x) = sin330°
    100° - 2x = 330° + k . 360°
              -2x = 230° + k . 360°
                 x = -115° - k . 360°
                 x = 245°
    100° - 2x = (180° - 330°) + k . 360°
              -2x = -150° + k . 360°
                 x = 75° - k . 360°
                 x = 75°
    HP = {75°, 245°}

c. (tan1/2x - tan50°)/(1 + tan1/2xtan50°) = -1
    tan(1/2x - 50°) = -1
    tan(1/2x - 50°) = tan135°
    1/2x - 50° = 135° + k . 180°
    1/2x = 185° + k . 180°
         x = 370° + k . 360°
         x = 10°
    HP = {10°}

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