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प्रश्न
Prove that cos(π + θ) = − cos θ
उत्तर
L..H.S = cos(π + θ)
= cos(180° + θ)
= cos 180° cos θ – sin 180° sin θ
= (– 1) cos θ – (0) sin θ
= – cos θ
= R.H.S
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