1.

सिद्ध करें कि : `tan^(-1)""(1)/(4) + tan^(-1)""(2)/(9)=(1)/(2) cos^(-1)((3)/(5))`

Answer» (1) में हम साबित कर चुके है कि
`tan^(-1)""(1)/(4) tan^(-1) ""(2)/(9) =tan^(-1)""(1)/(2) therefore L.H.S. = tan^(-1)""(1)/(2) = (1)/(2) *tan^(-1)""(1)/(2)`
` = (1)/(2) cos^(-1) ((1-(1)/(4))/(1 +(1)/(4)))[because 2tan^(-1) x = c os^(-1)(1-x^(2))/(1+x^(2)),` यहाँ `x = (1)/(2)`
` = (1)/(2) cos^(-1) ((3)/(5)) = R.H.S.`


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