In a right-angled triangle, one of the legs is 4 cm, and the cosine of the included angle is 0.8. Find the hypotenuse.

In a right-angled triangle, the cosine of an acute angle is equal to the ratio of the length of the adjacent leg to the length of its hypotenuse.

CosABC = BC / AB.

AB = BC / CosABC = 4 / 0.8 = 5 cm.

Answer: the length of the hypotenuse AB is 5 cm.

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