The identity: tan2a = cos2a1 - 1; tan2b = cos2b1 - 1;
⟹ cos2a1 - 1 = 1 + 2(cos2b1 - 1);
⟹ cos2a1 = cos2b2 ; cos2b = 2cos2a;
The identity: cos2b = 21+cos2b; cos2a = 21+cos2a ;
⟹ 21+cos2b = 1 + cos2a;
⟹ cos2b = 1 + 2cos2a.
Answer: from the relation tan2a = 1 + 2tan2b, follows: cos2b = 1 + 2cos2a.
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