Prove that cos 2theta - cos 4theta + cos 6theta - cos 12theta = 4 sin 3theta sin 5theta cos 4theta
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A fisherman can row upstream at 4 mph and downstream at 6 mph. He started rowing upstream until he got tired and then rowed downstream to his starting point. The distance of how far the fisherman row if the entire trip took 2 hours is 9.6 miles
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The speed of an object refers to the change of the object's distance within a specified time.
Mathematically, we can have;
Distance = speed × time
From the information given;
- The fisherman row upstream for 4 miles/ hour.
- The distance covered by the fisherman = 4 × t₁ ----- (1)
- The fisherman row downstream at 6 miles/ hour
- The distance covered by the fisherman = 6 × t₂ ------ (2)
- SInce it took the entire trip 2 hours, we can infer that:
- t₁ + t₂ = 2
- t₂ = 2 - t₁
From equation (2), replace the above value of t₂ into equation (2)
- d = 6 t₂
- d = 6(2 - t₁)
- d = 12 - 6t₁
From equation (1)
- d = 4 t₁
Equation both distance together, we have:
- 12 - 6t₁ = 4t₁
- 12 = 4t₁ + 6t₁
- 12 = 10t₁
- t₁ = 12/10
- t₁ = 1.2 hours
From t₂ = 2 - t₁
- t₂ = 2 - 1.2
- t₂ = 0.8 hours
From d = 4t₁
Replace t₁ with 1.2 hours
- d = 4(1.2 hours)
- d = 4.8 miles
Also, from d = 6t₂
- d = 6(0.8)
- d = 4.8 miles
Therefore, we can conclude that the distance of how far the fisherman row if the entire trip took 2 hours is (4.8+4.8) miles = 9.6 miles
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