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Exercise 4.3 · Q2

Q.Find the value of

(i) tan⁡−1(tan⁡5π4)\tan^{-1}\left(\tan\dfrac{5\pi}4\right)
(ii) tan⁡−1(tan⁡(−π6))\tan^{-1}\left(\tan\left(-\dfrac{\pi}6\right)\right).
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✓ Free question

tan⁡−1(tan⁡θ)=θ\tan^{-1}(\tan\theta)=\theta only when θ∈(−π2,π2)\theta\in\left(-\dfrac{\pi}2,\dfrac{\pi}2\right); when it isn't, we subtract the right multiple of π\pi (tan's period) to land back in range.

Step 1. (i) Check the range of θ=5π4\theta=\dfrac{5\pi}4. 5π4∉(−π2,π2)\dfrac{5\pi}4\notin\left(-\dfrac{\pi}2,\dfrac{\pi}2\right).

Step 2. (i) Shift by −π-\pi (tan has period π\pi). 5π4−π=π4∈(−π2,π2)\dfrac{5\pi}4-\pi=\dfrac{\pi}4\in\left(-\dfrac{\pi}2,\dfrac{\pi}2\right), and tan⁡5π4=tan⁡π4\tan\dfrac{5\pi}4=\tan\dfrac{\pi}4 since they differ by exactly π\pi.

Step 3. (i) Conclude. tan⁡−1(tan⁡5π4)=tan⁡−1(tan⁡π4)=π4\tan^{-1}\left(\tan\dfrac{5\pi}4\right)=\tan^{-1}\left(\tan\dfrac{\pi}4\right)=\dfrac{\pi}4.

Step 4. (ii) Check the range of θ=−π6\theta=-\dfrac{\pi}6. −π6∈(−π2,π2)-\dfrac{\pi}6\in\left(-\dfrac{\pi}2,\dfrac{\pi}2\right) already.

Step 5. (ii) Cancel directly. tan⁡−1(tan⁡(−π6))=−π6\tan^{-1}\left(\tan\left(-\dfrac{\pi}6\right)\right)=-\dfrac{\pi}6.

✓Final answer

(i) π4\dfrac{\pi}4. (ii) −π6-\dfrac{\pi}6.

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