Q4 of 164 Page 44

Show that

cos 20° cos 70° – sin 70° sin 20° = 0


Simplifying the left hand side:

⇒ cos 20° cos 70° – cos(90–70)° cos (90–20)°


⇒ cos 20° cos 70° – cos 20° cos 70°


⇒ 0


Which is equal to right hand side.


Hence proved.


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