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Exercise 4.1 · Q1

Q.Find all the values of xx such that

(i) −10π≤x≤10π-10\pi \le x \le 10\pi and sin⁡x=0\sin x = 0
(ii) −3π≤x≤3π-3\pi \le x \le 3\pi and sin⁡x=−1\sin x = -1.
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✓ Free question

We use the general solutions sin⁡x=0⇒x=kπ\sin x=0\Rightarrow x=k\pi and sin⁡x=−1⇒x=2kπ−π2\sin x=-1\Rightarrow x=2k\pi-\dfrac{\pi}2, then pick out every value that falls inside the stated interval.

Step 1. (i) Write the general solution of sin⁡x=0\sin x=0. sin⁡x=0  ⟺  x=kπ, k∈Z\sin x=0 \iff x=k\pi,\ k\in\mathbb{Z}.

Step 2. (i) Restrict to −10π≤x≤10π-10\pi\le x\le10\pi. We need −10π≤kπ≤10π  ⟺  −10≤k≤10-10\pi\le k\pi\le10\pi \iff -10\le k\le10. So k=−10,−9,…,9,10k=-10,-9,\dots,9,10, giving the 2121 values x=−10π,−9π,…,9π,10πx=-10\pi,-9\pi,\dots,9\pi,10\pi.

Step 3. (ii) Write the general solution of sin⁡x=−1\sin x=-1. sin⁡x=−1  ⟺  x=2kπ−π2, k∈Z\sin x=-1 \iff x=2k\pi-\dfrac{\pi}2,\ k\in\mathbb{Z}.

Step 4. (ii) Restrict to −3π≤x≤3π-3\pi\le x\le3\pi. Testing consecutive kk: k=0⇒x=−π2k=0\Rightarrow x=-\dfrac{\pi}2 (in range); k=1⇒x=3π2k=1\Rightarrow x=\dfrac{3\pi}2 (in range); k=−1⇒x=−5π2k=-1\Rightarrow x=-\dfrac{5\pi}2 (in range, since −3π≈−9.42<−5π2≈−7.85-3\pi\approx-9.42<-\dfrac{5\pi}2\approx-7.85); k=2⇒x=7π2≈11.0>3π≈9.42k=2\Rightarrow x=\dfrac{7\pi}2\approx11.0>3\pi\approx9.42 (out); k=−2⇒x=−9π2≈−14.1<−3πk=-2\Rightarrow x=-\dfrac{9\pi}2\approx-14.1<-3\pi (out).

Step 5. Collect the valid values. So for (ii): x=−5π2, −π2, 3π2x=-\dfrac{5\pi}2,\ -\dfrac{\pi}2,\ \dfrac{3\pi}2.

✓Final answer

(i) x=kπ, k=−10,−9,…,9,10x=k\pi,\ k=-10,-9,\dots,9,10 (21 values). (ii) x=−5π2, −π2, 3π2x=-\dfrac{5\pi}2,\ -\dfrac{\pi}2,\ \dfrac{3\pi}2.

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