1.

सिद्ध करें कि: `tan^(-1)""(1)/(4)+tan^(-1)""(2)/(9)=(1)/(2)tan^(-1)""(4)/(3)`

Answer» `L.H.S. = tan^(-1)((1)/(4))+tan^(-1)((2)/(9))`
` = tan^(-1)[((1)/(4)+(2)/(9))/(1-((1)/(4))((2)/(9)))] [because tan^(-1)x + tan^(-1) y = tan^(-1)""(x+y)/(1-xy)]`
`= tan^(-1)[((9+8)/(36))/((36-2)/(36))]=tan^(-1)((17)/(34)) = tan^(-1)((1)/(2))" "...(1)`
` = (1)/(2) tan^(-1) ""(2((1)/(2)))/(1-((1)/(2))^(2))" "[because tan^(-1) x = (1)/(2) (2x)/(1-x^(2))]`
` = (1)/(2) tan^(-1) ""(1)/(1-(1)/(4)) = (1)/(2) tan^(-1) ""(1)/(3//4) = (1)/(2) tan^(-1) ""(4)/(3) = R.H.S.`


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