1.

Prove that: `2tan^(-1)(1/2)+tan^(-1)(1/7)=tan^(-1)(31/17)`

Answer» We know, `tan^-1x+tan-^-1y = tan^-1((x+y)/(1-xy))`
Here, `2tan^-1(1/2)` can be written as:
`tan^-1(1/2)+tan^-1(1/2) = tan^-1((1/2+1/2)/(1-1/2*1/2)) =tan-1(4/3) `
`L.H.S. = 2tan^-1(1/2)+tan^-1(1/7)`
`=tan^-1(4/3)+tan^-1(1/7) = tan^-1((4/3+1/7)/(1-4/3*1/7))`
`=tan^-1((31/21)/(17/21)) = tan^-1(31/17) = R.H.S.`


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