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Exercise 4.1 · Q3

Q.Sketch the graph of y=sin⁡(13x)y = \sin\left(\dfrac13 x\right) for 0≤x<6π0 \le x < 6\pi.

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Since the period of y=sin⁡(13x)y=\sin\left(\dfrac13x\right) is 6π6\pi (Q2(ii)), the interval 0≤x<6π0\le x<6\pi covers exactly one cycle; we tabulate key points and connect them with the usual sine shape.

Step 1. Confirm the period and amplitude. As in Q2(ii), y=sin⁡(13x)y=\sin\left(\dfrac13x\right) has amplitude 11 and period 2π1/3=6π\dfrac{2\pi}{1/3}=6\pi, so 0≤x<6π0\le x<6\pi is precisely one cycle.

Step 2. Build a table of key points. Let u=x3u=\dfrac{x}3, so uu runs over [0,2π)[0,2\pi) as xx runs over [0,6π)[0,6\pi).

xx: 0, 3π2, 3π, 9π2, 6π−0,\ \dfrac{3\pi}2,\ 3\pi,\ \dfrac{9\pi}2,\ 6\pi^{-} correspond to u=x3u=\dfrac x3: 0, π2, π, 3π2, 2π−0,\ \dfrac{\pi}2,\ \pi,\ \dfrac{3\pi}2,\ 2\pi^-

y=sin⁡uy=\sin u: 0, 1, 0, −1, →00,\ 1,\ 0,\ -1,\ \to0

Step 3. Describe the curve. Starting at the origin (0,0)(0,0), the graph rises smoothly to its maximum (3π2,1)(\tfrac{3\pi}2,1), falls back through (3π,0)(3\pi,0) to its minimum (9π2,−1)(\tfrac{9\pi}2,-1), and rises back toward 00 as x→6π−x\to6\pi^- — one full smooth sine wave, stretched horizontally by a factor of 33 compared to y=sin⁡xy=\sin x.

Step 4. Plot and join. Plotting these five points and joining them with the usual smooth sine shape gives the required sketch over 0≤x<6π0\le x<6\pi.

✓Final answer

One complete sine cycle rising 0→1→0→−1→00\to1\to0\to-1\to0 at x=0,3π2,3π,9π2,6πx=0,\dfrac{3\pi}2,3\pi,\dfrac{9\pi}2,6\pi respectively (amplitude 11, period 6π6\pi).

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