# In triangle ABC, angle C = 90, AC = 15, cosA = 5/7. Find AB

June 29, 2021 | education

| A triangle is three points that do not lie on one straight line, connected by segments. In this case, the points are called the vertices of the triangle, and the segments are called its sides.

A rectangular triangle is a triangle in which one corner is a straight line (equal to 90º). The side opposite to the right angle is called the hypotenuse, and the other two are called the legs.

To calculate the length of the hypotenuse AB, we apply the cosine theorem

The cosine of an acute angle of a right-angled triangle is the ratio of the adjacent leg to the hypotenuse:

cos A = AC / AB;

AB = AC / cos A;

AB = 15: 5/7 = 15 7/5 = 105/5 = 21 cm.

Answer: the length of the hypotenuse AB is 21 cm.

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