# Question Video: Finding the Cosine of Angles in Right-Angled Triangles given the Opposite Side and the Hypotenuse Mathematics • 10th Grade

Find cos ๐ด, given ๐ด๐ต๐ถ is a right-angled triangle at ๐ถ where ๐ด๐ต = 10 cm and ๐ต๐ถ = 6 cm.

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### Video Transcript

Find cos of ๐ด, given ๐ด๐ต๐ถ is a right-angled triangle at ๐ถ, where ๐ด๐ต is equal to 10 centimeters and ๐ต๐ถ is equal to six centimeters.

This question is asking us to find the cos or cosine of an angle ๐ด. The definition of the cosine or cos of an angle ๐ in a right-angled triangle is the ratio between the adjacent side and the hypotenuse of the triangle. Letโs recall what these terms mean.

The hypotenuse in any right-angled triangle is the side directly opposite the right angle. Itโs also the longest side of the triangle. The side opposite the angle weโre interested in โ so in this case thatโs angle ๐ด โ is called the opposite. The third side of the triangle which is between the angle weโre interested in and the right angle is called the adjacent. This means that in our triangle cos of the angle ๐ด is equal to ๐ด๐ถ, the adjacent side, divided by ๐ด๐ต, the hypotenuse.

Weโve been given the length of the hypotenuse is 10 centimeters. So we have that cos of angle ๐ด is equal to ๐ด๐ถ over 10. In order to find cos of angle ๐ด, we need to work out the length of ๐ด๐ถ. Notice that weโve been given the length of both of the other two sides of the triangle. And as the triangle is right angled, we can apply the Pythagorean theorem to calculate the length of the third side.

The Pythagorean theorem tells us that in a right-angled triangle, the sum of the squares of the two shorter sides is equal to the square of the hypotenuse. In this triangle, this means that ๐ด๐ถ squared plus ๐ต๐ถ squared is equal to ๐ด๐ต squared. ๐ต๐ถ is six centimeters and ๐ด๐ต is 10 centimeters. So we have the equation ๐ด๐ถ squared plus six squared is equal to 10 squared. And we can solve this equation to find the length of ๐ด๐ถ.

Six squared is equal to 36 and 10 squared is equal to 100. So we have ๐ด๐ถ squared plus 36 is equal to 100. Subtracting 36 from each side of the equation, we have that ๐ด๐ถ squared is equal to 64. To find the value of ๐ด๐ถ, we need to take the square root. 64 is a squared number. So its square root is just eight.

Now, you may have been able to spot this without going through all the working out for the Pythagorean theorem if youโre familiar with your Pythagorean triples as six, eight, 10 is an example of one. A Pythagorean triple is a right-angled triangle in which the length of all three sides are integers. In fact, the six, eight, 10 triangle is just an enlargement of the three, four, five triangle, which is probably the most commonly recognized Pythagorean triple.

Whether you spot it straight away or whether you had to go through the working out, we now know that the length of ๐ด๐ถ the adjacent side is eight centimeters. So can substitute this into our formula for cos of ๐ด. We have that cos of ๐ด is equal to eight over 10. This fraction can be simplified slightly as both the numerator and denominator are multiples of two. So dividing them both by two, we have that cos of ๐ด is equal to four-fifths.