Answer :
Formula: - (i) 
(ii) tan(90° – A) = cot A
Taking formula (i)
⇒ tanA – tanB = tan(A – B)(1 + tanAtanB)
⇒ tan70° – tan20° = tan(70° – 20°)(1 + tan70°tan20°)
⇒ tan70° – tan20° = tan50°(1 + tan70°cot70°) - - - - from formula (ii)
⇒ tan70° – tan20° = 2tan50°
L.H.S = R.H.S
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