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Exercise 4.2 · Q1

Q.Find all values of xx such that

(i) −6π≤x≤6π-6\pi \le x \le 6\pi and cos⁡x=0\cos x = 0
(ii) −5π≤x≤5π-5\pi \le x \le 5\pi and cos⁡x=1\cos x = 1.
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✓ Free question

We use the general solutions cos⁡x=0⇒x=(2k+1)π2\cos x=0\Rightarrow x=(2k+1)\dfrac{\pi}2 and cos⁡x=1⇒x=2kπ\cos x=1\Rightarrow x=2k\pi, then restrict kk to the given interval.

Step 1. (i) General solution of cos⁡x=0\cos x=0. cos⁡x=0  ⟺  x=(2k+1)π2, k∈Z\cos x=0\iff x=(2k+1)\dfrac{\pi}2,\ k\in\mathbb{Z}.

Step 2. (i) Restrict to −6π≤x≤6π-6\pi\le x\le6\pi. Need −6π≤(2k+1)π2≤6π⇒−12≤2k+1≤12⇒−6.5≤k≤5.5-6\pi\le(2k+1)\dfrac{\pi}2\le6\pi\Rightarrow-12\le2k+1\le12\Rightarrow-6.5\le k\le5.5, so k=−6,−5,…,5k=-6,-5,\dots,5 — that is 1212 integer values, giving x=−11π2,−9π2,…,9π2,11π2x=-\dfrac{11\pi}2,-\dfrac{9\pi}2,\dots,\dfrac{9\pi}2,\dfrac{11\pi}2.

Step 3. (ii) General solution of cos⁡x=1\cos x=1. cos⁡x=1  ⟺  x=2kπ, k∈Z\cos x=1\iff x=2k\pi,\ k\in\mathbb{Z}.

Step 4. (ii) Restrict to −5π≤x≤5π-5\pi\le x\le5\pi. Need −5π≤2kπ≤5π⇒−2.5≤k≤2.5-5\pi\le2k\pi\le5\pi\Rightarrow-2.5\le k\le2.5, so k=−2,−1,0,1,2k=-2,-1,0,1,2, giving x=−4π,−2π,0,2π,4πx=-4\pi,-2\pi,0,2\pi,4\pi.

✓Final answer

(i) x=(2k+1)π2, k=−6,…,5x=(2k+1)\dfrac{\pi}2,\ k=-6,\dots,5 (12 values). (ii) x=2kπ, k=−2,−1,0,1,2x=2k\pi,\ k=-2,-1,0,1,2 (5 values).

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