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The diagram below shows the graphs of f(x) = 2sin 2x and g(x) = a tan bx. Give the coordinates of A and B. Write down the coordinates of C and D. What are the values of a and b?

Mathematics

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Step-by-step solution

4 steps
  1. Determine the coordinates of A and B.

    Points A and B are the x-intercepts of the function f(x)=2sin⁡(2x)f(x) = 2\sin(2x). At x-intercepts, f(x)=0f(x) = 0.

    2sin⁡(2x)=02\sin(2x) = 0 sin⁡(2x)=0\sin(2x) = 0

    This occurs when 2x=n⋅180∘2x = n \cdot 180^\circ for any integer nn.

    x=n⋅90∘x = n \cdot 90^\circ

    From the graph, A is the first x-intercept to the left of the origin, so n=−1n=-1, which gives x=−90∘x = -90^\circ.
    B is the first x-intercept to the right of the origin, so n=1n=1, which gives x=90∘x = 90^\circ.
    The coordinates are:
    A: (−90∘,0)\boxed{(-90^\circ, 0)}
    B: (90∘,0)\boxed{(90^\circ, 0)}

  2. Determine the coordinates of C and D.

    Point C is a local maximum of f(x)=2sin⁡(2x)f(x) = 2\sin(2x). The maximum value of 2sin⁡(2x)2\sin(2x) is 2.
    This occurs when sin⁡(2x)=1\sin(2x) = 1.

    2x=90∘+k⋅360∘2x = 90^\circ + k \cdot 360^\circ x=45∘+k⋅180∘x = 45^\circ + k \cdot 180^\circ

    From the graph, C is the first maximum in the positive x-axis, so k=0k=0, which gives x=45∘x = 45^\circ.
    Point D is a local minimum of f(x)=2sin⁡(2x)f(x) = 2\sin(2x). The minimum value of 2sin⁡(2x)2\sin(2x) is -2.
    This occurs when sin⁡(2x)=−1\sin(2x) = -1.

    2x=−90∘+k⋅360∘2x = -90^\circ + k \cdot 360^\circ x=−45∘+k⋅180∘x = -45^\circ + k \cdot 180^\circ

    From the graph, D is the first minimum in the negative x-axis, so k=0k=0, which gives x=−45∘x = -45^\circ.
    The coordinates are:
    C: (45∘,2)\boxed{(45^\circ, 2)}
    D: (−45∘,−2)\boxed{(-45^\circ, -2)}

  3. Determine the values of aa and bb.

    The function is g(x)=atan⁡(bx)g(x) = a\tan(bx).
    From the graph, the vertical asymptotes of g(x)g(x) are at x=−90∘x = -90^\circ and x=90∘x = 90^\circ.
    For a tangent function, vertical asymptotes occur when the argument is 90∘+n⋅180∘90^\circ + n \cdot 180^\circ.
    So, bx=90∘bx = 90^\circ.
    Using x=90∘x = 90^\circ:

    b(90∘)=90∘b(90^\circ) = 90^\circ b=1b = 1

    Now, use point C (45∘,2)(45^\circ, 2), which lies on g(x)g(x). Substitute x=45∘x=45^\circ, y=2y=2, and b=1b=1 into g(x)g(x):

    2=atan⁡(1⋅45∘)2 = a\tan(1 \cdot 45^\circ) 2=atan⁡(45∘)2 = a\tan(45^\circ)

    Since tan⁡(45∘)=1\tan(45^\circ) = 1:

    2=a(1)2 = a(1) a=2a = 2

    The values are: a=2,b=1\boxed{a=2, b=1}

  4. Determine the values of xx for which f(x)>g(x)f(x) > g(x) if x∈[0∘;90∘]x \in [0^\circ; 90^\circ].

    We need to solve 2sin⁡(2x)>2tan⁡(x)2\sin(2x) > 2\tan(x) for x∈[0∘;90∘]x \in [0^\circ; 90^\circ].
    Divide by 2:

    sin⁡(2x)>tan⁡(x)\sin(2x) > \tan(x)

    Use the identities sin⁡(2x)=2sin⁡(x)cos⁡(x)\sin(2x) = 2\sin(x)\cos(x) and tan⁡(x)=sin⁡(x)cos⁡(x)\tan(x) = \frac{\sin(x)}{\cos(x)}:

    2sin⁡(x)cos⁡(x)>sin⁡(x)cos⁡(x)2\sin(x)\cos(x) > \frac{\sin(x)}{\cos(x)}

    For x∈(0∘,90∘)x \in (0^\circ, 90^\circ), both sin⁡(x)>0\sin(x) > 0 and cos⁡(x)>0\cos(x) > 0.
    Multiply both sides by cos⁡(x)\cos(x) (which is positive, so the inequality direction remains the same):

    2sin⁡(x)cos⁡2(x)>sin⁡(x)2\sin(x)\cos^2(x) > \sin(x)

    Divide both sides by sin⁡(x)\sin(x) (which is positive, so the inequality direction remains the same):

    2cos⁡2(x)>12\cos^2(x) > 1 cos⁡2(x)>12\cos^2(x) > \frac{1}{2}

    Since cos⁡(x)>0\cos(x) > 0 in the interval (0∘,90∘)(0^\circ, 90^\circ), take the positive square root:

    cos⁡(x)>12\cos(x) > \sqrt{\frac{1}{2}} cos⁡(x)>12\cos(x) > \frac{1}{\sqrt{2}} cos⁡(x)>22\cos(x) > \frac{\sqrt{2}}{2}

    We know that cos⁡(45∘)=22\cos(45^\circ) = \frac{\sqrt{2}}{2}. Since cos⁡(x)\cos(x) is a decreasing function in the interval (0∘,90∘)(0^\circ, 90^\circ), for cos⁡(x)\cos(x) to be greater than 22\frac{\sqrt{2}}{2}, xx must be less than 45∘45^\circ.
    Also, at x=0∘x=0^\circ, f(0)=0f(0)=0 and g(0)=0g(0)=0, so f(0)≯g(0)f(0) \ngtr g(0).
    At x=45∘x=45^\circ, f(45∘)=2f(45^\circ)=2 and g(45∘)=2g(45^\circ)=2, so f(45∘)≯g(45∘)f(45^\circ) \ngtr g(45^\circ).
    Therefore, the inequality holds for xx values strictly between 0∘0^\circ and 45∘45^\circ.
    The values of xx are: 0∘<x<45∘\boxed{0^\circ < x < 45^\circ}

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