Doubt from limit as sum

Answer is 12.5

done by jagdish sir before by 2 method 1st limit as a sum nd other by complex number method...

Answer should be 25..
Limit is coming out to be 1/2..

@Hritik_2020 ans coming out 25

For question 73 first use property of trigo identity sinAsin2ASin3A series and then apply Ap or gp properties