sin21°cos 9°– cos84°cos6°
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multiplying it by 2 ,
=2sin21°cos9° – 2cos84°cos6°
=cos(21–9)°– cos(21+9)° – cos (84+6) –cos(84–6)
=cos 12° – cos30° – cos90° – cos78°
=cos(90–78)°–√3/2 – 0 – cos 78°
=sin78° –0.87–cos 78°
=0.97–0.87– 0.20
=–0.1
=2sin21°cos9° – 2cos84°cos6°
=cos(21–9)°– cos(21+9)° – cos (84+6) –cos(84–6)
=cos 12° – cos30° – cos90° – cos78°
=cos(90–78)°–√3/2 – 0 – cos 78°
=sin78° –0.87–cos 78°
=0.97–0.87– 0.20
=–0.1
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