1.

`cos^(-1)""(4)/(5)+cos^(-1)""(12)/(13)=cos^(-1)""(33)/(65)`

Answer» `L.H.S.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)=R.H.S.`


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