Question

Tính giá trị biểu thức :

\(N=lg\left(\tan1^0\right)+lg\left(\tan2^0\right)+....+lg\left(\tan88^0\right)+lg\left(\tan89^0\right)\)

Answer 1

\(N=lg\left(\tan1^0\right)+lg\left(\tan2^0\right)+....+lg\left(\tan88^0\right)+lg\left(\tan89^0\right)\)

     \(=\left[lg\left(\tan1^0\right)+lg\left(\tan89^0\right)\right]+\left[lg\left(\tan2^0\right)+lg\left(\tan88^0\right)\right]+...+\left[lg\left(\tan44^0\right)+lg\left(\tan46^0\right)\right]+lg\left(\tan45^0\right)\)

     \(=lg\left(\tan1^0.\tan89^0\right)+lg\left(\tan2^0.\tan88^0\right)+...+lg\left(\tan44^0.\tan46^0\right)+lg\left(\tan45^0\right)\)

     \(=lg\left(\tan1^0.\cot1^0\right)+lg\left(\tan2^0.\cot2^0\right)+.....+lg\left(\tan44^0.\cot44^0\right)+lg\left(\tan45^0\right)\)

     \(=lg1+lg1+....+lg1+lg1=0+0+....+0+0=0\)