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Evaluate Tan Alpha for Sum of Inverse Tangent Series

If π4+p=111tan1(2p11+22p1)=α\frac{\pi}{4} + \sum_{p=1}^{11} \tan^{-1}\left(\frac{2^{p-1}}{1 + 2^{2p-1}}\right) = \alpha, then tanα\tan \alpha is equal to _________.

Official Numerical Answer2048

Step-by-Step Solution

To evaluate tanα\tan \alpha, we first simplify the general term of the given summation.

The given equation is: α=π4+p=111tan1(2p11+22p1)\alpha = \frac{\pi}{4} + \sum_{p=1}^{11} \tan^{-1}\left(\frac{2^{p-1}}{1 + 2^{2p-1}}\right)

Let TpT_p denote the pp-th term of the summation: Tp=tan1(2p11+22p1)T_p = \tan^{-1}\left(\frac{2^{p-1}}{1 + 2^{2p-1}}\right)

Notice that 22p1=2p2p12^{2p-1} = 2^p \cdot 2^{p-1} and 2p1=2p2p12^{p-1} = 2^p - 2^{p-1}. We can rewrite TpT_p as: Tp=tan1(2p2p11+2p2p1)T_p = \tan^{-1}\left(\frac{2^p - 2^{p-1}}{1 + 2^p \cdot 2^{p-1}}\right)

Using the standard inverse trigonometric identity: tan1(xy1+xy)=tan1xtan1yfor x,y>0\tan^{-1}\left(\frac{x - y}{1 + xy}\right) = \tan^{-1}x - \tan^{-1}y \quad \text{for } x, y > 0

we get: Tp=tan1(2p)tan1(2p1)T_p = \tan^{-1}(2^p) - \tan^{-1}(2^{p-1})

Now, summing TpT_p from p=1p = 1 to 1111 forms a telescoping series: p=111Tp=p=111[tan1(2p)tan1(2p1)]\sum_{p=1}^{11} T_p = \sum_{p=1}^{11} \left[ \tan^{-1}(2^p) - \tan^{-1}(2^{p-1}) \right]

Expanding the sum: p=111Tp=(tan1(21)tan1(20))+(tan1(22)tan1(21))++(tan1(211)tan1(210))\sum_{p=1}^{11} T_p = \left( \tan^{-1}(2^1) - \tan^{-1}(2^0) \right) + \left( \tan^{-1}(2^2) - \tan^{-1}(2^1) \right) + \dots + \left( \tan^{-1}(2^{11}) - \tan^{-1}(2^{10}) \right)

All intermediate terms cancel out, leaving: p=111Tp=tan1(211)tan1(20)\sum_{p=1}^{11} T_p = \tan^{-1}(2^{11}) - \tan^{-1}(2^0)

Since 20=12^0 = 1 and tan1(1)=π4\tan^{-1}(1) = \frac{\pi}{4}: p=111Tp=tan1(211)π4\sum_{p=1}^{11} T_p = \tan^{-1}(2^{11}) - \frac{\pi}{4}

Substitute this result back into the expression for α\alpha: α=π4+(tan1(211)π4)\alpha = \frac{\pi}{4} + \left( \tan^{-1}(2^{11}) - \frac{\pi}{4} \right) α=tan1(211)\alpha = \tan^{-1}(2^{11})

Taking the tangent on both sides: tanα=tan(tan1(211))=211\tan \alpha = \tan\left(\tan^{-1}(2^{11})\right) = 2^{11}

Since 211=20482^{11} = 2048: tanα=2048\tan \alpha = 2048

Evaluate Tan Alpha for Sum of Inverse Tangent Series | Mathematics PYQ Solution - JEE Challenger