Lesson video

Hi.

I'm Mrs. Dennett.

And in this lesson, you're going to be needing a scientific calculator.

Make sure it's set in degrees mode because we're going to be using it to help us use inverse functions to find a missing angle.

We are asked to work out the size of the angle theta to one decimal place.

We label the sides of our triangle with hypotenuse, opposite and adjacent.

H, O and A.

Remember, the hypotenuse is the longest side directly opposite the right angle.

And the opposite side is the side opposite the angle we're trying to find.

So the opposite will be six centimetres.

Adjacent is the remaining side.

In this question, we're given the opposite and adjacent side lengths.

So this tells us that we're going to be using the Tan ratio.

We write Tan theta equals 6/8.

Make sure you put the opposite side on top of the fraction.

Then we have to find the inverse.

We'll need our scientific calculator for this part.

Press Shift + Tan for inverse Tan or Tan to the minus one.

6/8 or six divided by eight and equals to get theta equals 36.

869 et cetera.

And round to one decimal place, so theta is equal to 36.

9 degrees.

Let's try another example.

This time we have the hypotenuse and adjacent sides.

So we use Cosine.

We write Cos theta equals 4/15.

Use inverse Cosine to find the angle and round to one decimal place.

So theta is equal to 74.

5 degrees.

Here are some questions for you to try.

Pause the video to complete the task and restart when you have finished.

Here are the answers.

We've got each step of the calculation and you just need to fill in the blanks.

So make sure you put the sidelines in the correct positions in the fractions.

It's a really common error to get these mixed up.

So make sure that when you're doing Sine theta, you're doing the opposite over the hypotenuse.

When you're doing Cos theta, you're doing the adjacent over the hypotenuse side.

And when you're using Tan, you're using the opposite over the adjacent side.

Check that you're always using the inverse function to find an angle.

Here are some questions for you to try.

Pause the video to complete the task and restart when you have finished.

Here are the answers.

To help you check, I'm going to run through each question and tell you which trigonometry ratio, which trigonometry function you should have been using.

And you should be using inverse functions for all of these because we're finding angles.

So for part A, you should have been using Cos to minus one.

For part B, you need the Sin to the minus one function.

For part C, you need inverse Tan or Tan to the minus one function.

And finally, for part D, you need Sin to the minus one again.

If you still making mistakes and you're seeing really different in terms of your answers, make sure that you calculator is still in degrees mode.

You should see a little D at the top of your calculator to display.

And this indicates that your calculator is working in degrees.

Here's a final question for you to try.

Pause the video to complete the task and restart when you have finished.

In this question, you should hopefully have spotted that Jordan has used the Cosine function instead of the Sine function.

He should have written Sin theta equals 9/12 and then used in the Sine to find the angle.

That's all for this lesson.

Remember to take the exit quiz before you leave.

Thank you for watching.