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# Help with the Cosine rule (GCSE)? watch

1. Today I have been doing a past GCSE Maths paper and one of the questions has really stumped me. Not because I don't know what to do, but I can't seem to get the right answer!

In the question I have to calculate a length, AB, which is part of a triangle ABC. Effectively it is side c as it's opposite to angle C which is 75. The other information I have is that the other side lengths are 5 and 8cm. (sorry I can't provide it visually, can't find it online)

I thought the formula should go like this:
c^2=b^2+a^2-2ab cos C

And therefore this:
c^2=25+64-2x8x5 cos 75

When I input it into my calculator I get c^2 = 2.33cm (3 s.f.)
This can't be right, can it? On the diagram it is shown significantly larger than the side with length 5cm and I know they are not drawn "accurately" but still. Please help, this is driving me mad!! Am I inputting it wrongly? Many thanks in advance.
2. How did you get that? What did you type into your calculator? is correct, and the RHS certainly doesn't evaluate to 2.33.
3. Are you sure you typed it in your calculator correctly? I get a different answer, c^2=68.29 so c=8.26 which fits.
4. (Original post by ghostwalker)
Is your calculator in radians mode (incorrectly)?
Even if it was it wouldn't give anything near 2.33.
5. (Original post by nuodai)
How did you get that? What did you type into your calculator? is correct, and the RHS certainly doesn't evaluate to 2.33.
Thanks for the confirmation that my working is correct. I think I am typing it in wrong, as I've tried this on two different calculators now. I typed it in all in one go (as in, with no brackets or anything) up until the cos 75 part. This gave me the answer 9, which multiplied by cos 75 gives me 2.33. I multiplied it by cos by inputting the multiplication sign, then the number 75, then cos. I am extremely confused

edit: it is not in radians mode
6. You need to do 89-(80*cos 75). Make sure you process the bracket first, otherwise you will get 2.33, not 8.26
7. (Original post by jameswhughes)
You need to do 89-(80*cos 75). Make sure you process the bracket first, otherwise you will get 2.33, not 8.26
Thank you so much, I knew there was something I was doing wrong!! This is a huge help, thanks.
8. (Original post by Where'sPerry?)
Thanks for the confirmation that my working is correct. I think I am typing it in wrong, as I've tried this on two different calculators now. I typed it in all in one go (as in, with no brackets or anything) up until the cos 75 part. This gave me the answer 9, which multiplied by cos 75 gives me 2.33. I multiplied it by cos by inputting the multiplication sign, then the number 75, then cos. I am extremely confused

edit: it is not in radians mode
Ah I see. What you did was work out and then multiply it by , so that gives , which is equal to (by expanding the bracket). What you actually want is .

What you need to do is type it all in in one go, including the bit. Alternatively, work out , then multiply that by , and then add on .

[EDIT: Basically what I'm saying is that you have to follow BIDMAS.]
9. (Original post by Where'sPerry?)
Today I have been doing a past GCSE Maths paper and one of the questions has really stumped me. Not because I don't know what to do, but I can't seem to get the right answer!

In the question I have to calculate a length, AB, which is part of a triangle ABC. Effectively it is side c as it's opposite to angle C which is 75. The other information I have is that the other side lengths are 5 and 8cm. (sorry I can't provide it visually, can't find it online)

I thought the formula should go like this:
c^2=b^2+a^2-2ab cos C

And therefore this:
c^2=25+64-2x8x5 cos 75

When I input it into my calculator I get c^2 = 2.33cm (3 s.f.)
This can't be right, can it? On the diagram it is shown significantly larger than the side with length 5cm and I know they are not drawn "accurately" but still. Please help, this is driving me mad!! Am I inputting it wrongly? Many thanks in advance.
Put the whole thing in at once. I had this problem before.

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