A man observes that when he has walked $c$ metres up an incline, the angular depression of an object in a horizontal plane through the foot of the slope is $\alpha$, and that when he has walked a further distance of $c$ metres the depression is $\beta$.then the inclination of the slope to the horizon is the angle whose cotangent is $\left( 4\cot { \alpha } -\cot { \beta } \right)$.
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