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d. A wave moving in one direction reflects at a barrier and interferes with the incoming wave moving in the opposite direction. If the displacements of the first and second waves are given as:

[tex]\[ y_1 = 25 \sin (15t - 6x) \][/tex]
[tex]\[ y_2 = 25 \sin (15t + 6x) \][/tex]

(i) Find the resulting wave [tex]\((y_1 + y_2)\)[/tex]. (1.5 marks)

(ii) If in (i) above [tex]\( y_1 = 4 \cos (2t - 30) \)[/tex] and [tex]\( y_2 = -3 \sin (2t + 60) \)[/tex], then find the new resulting wave. (1.5 marks)

Sagot :

Let's solve the problem step-by-step:

### Part (i)

Given:
- [tex]\( y_1 = 25 \sin (15t - 6x) \)[/tex]
- [tex]\( y_2 = 25 \sin (15t + 6x) \)[/tex]

We need to find the resulting wave [tex]\(y_{\text{result}} = y_1 + y_2\)[/tex].

To do this, we use a trigonometric identity:
[tex]\[ \sin(A) + \sin(B) = 2 \sin\left(\frac{A + B}{2}\right) \cos\left(\frac{A - B}{2}\right) \][/tex]

For our waves:
- Let [tex]\( A = 15t - 6x \)[/tex]
- Let [tex]\( B = 15t + 6x \)[/tex]

Plugging these into the identity, we get:
[tex]\[ y_1 + y_2 = 25 \sin (15t - 6x) + 25 \sin (15t + 6x) \][/tex]

Let's express it using the trigonometric identity:
[tex]\[ \sin(15t - 6x) + \sin(15t + 6x) = 2 \sin\left( \frac{(15t - 6x) + (15t + 6x)}{2} \right) \cos\left( \frac{(15t - 6x) - (15t + 6x)}{2} \right) \][/tex]
[tex]\[ = 2 \sin\left( \frac{30t}{2} \right) \cos\left( \frac{-12x}{2} \right) \][/tex]
[tex]\[ = 2 \sin(15t) \cos(6x) \][/tex]

Thus,
[tex]\[ 25 \sin(15t - 6x) + 25 \sin(15t + 6x) = 2 \times 25 \sin(15t) \cos(6x) \][/tex]
[tex]\[ = 50 \sin(15t) \cos(6x) \][/tex]

So, the resulting wave is:
[tex]\[ y_{\text{result}} = 50 \sin (15t) \cos (6x) \][/tex]

### Part (ii)

Given:
- [tex]\( y_1 = 4 \cos (2t - 30) \)[/tex]
- [tex]\( y_2 = -3 \sin (2t + 60) \)[/tex]

We need to find the resulting wave [tex]\(y_{\text{result}} = y_1 + y_2\)[/tex].

So the resulting wave is simply the sum of the two given displacements:
[tex]\[ y_{\text{result}} = 4 \cos (2t - 30) + (-3 \sin (2t + 60)) \][/tex]
[tex]\[ y_{\text{result}} = 4 \cos (2t - 30) - 3 \sin (2t + 60) \][/tex]

In conclusion, the resulting waves are:
1. For part (i):
[tex]\[ y_{\text{result}} = 50 \sin (15t) \cos (6x) \][/tex]

2. For part (ii):
[tex]\[ y_{\text{result}} = 4 \cos (2t - 30) - 3 \sin (2t + 60) \][/tex]
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