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if cosФ<0 and tg Ф>0; then we are in the third quadrant, then sin Ф<0 Fundamental realtion of trygonometry: sin²Ф+cos²Ф=1 Then: sin Ф=⁺₋√(1-cos₂Ф): Because we are in the third quadrant; sin Ф=-√(1-cos²Ф) Sin Ф=-√(1-(-2/5)²)=-√(1- 4/25)=-√(21/25)=-√21 / 5 Answer: sin Ф=-√21 / 5
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