1.

Prove that `cos^(-1)4/5cos^(-1)(12)/(13)=cos^(-1)(33)/(65)`

Answer» LHS`=cos^(-1)""(4)/(5)+cos^(-1)""(12)/(13)`
`=cos^(-1)[(4)/(5)xx(12)/(13)-sqrt(1-((4)/(5))^(2))sqrt(1-((12)/(13))^(2))]`
`=cos^(-1)[(48)/(65)-(3)/(5)xx(5)/(13)]=cos^(-1)((48)/(65)-(15)/(65))`
`=cos^(-1)""(33)/(65)=` RHS. Hence Proved.


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