sin (70°+o)cos (20°-o)=0
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Answer:
sin (70)- cos(20)
sin (70)- cos(90-70)
sin (70)- sin (70)
=0
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[sin(70) cos0 + cos(70) sin0][cos(20) cos0 - sin(20) sin0] = 0
[sin (70) × 1 + cos (70) × 0][cos (20) × 1 - sin (20) × 0] = 0
(sin 70 )(cos 20) = 0
sin(90-20) cos (20) = 0
(cos 20)(cos 20) = 0
cos²20 = 0
=> cos 20 = 0
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