The angle θ to the diameter that she should row is; θ = 56.44°
How to find the angle of an arc?
It should be noted that;
a = 90° - θ. Thus;
b = 2θ
For any circle, the angle subtended (in radians) is equal to the arc length (d_w ) divided by the radius (1.2). Thus; d_w = 4θ .
The time required to walk this distance is given by;
t_w = 4θ/5 = 0.8θ
The distance rowed is given by: cos θ = d_r/2.4
d_r = 2.4cos θ while the time needed for it is;
t_r = 2.4cos θ/3.5
t_r = 0.6857θ
Thus, total time is;
T(θ) = t_r + t_w
T(θ) = 0.6857θ + 0.8θ
T(θ) = 1.4857θ
Now we need the critical numbers:
T'(θ) = 1 - 1.2 sin θ
At T'(θ) = 0, we have;
1 - 1.2 sin θ = 0
1/1.2 = sin θ
θ = sin⁻¹(1/1.2)
θ = 56.44°
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