Here’s the boards from the lesson today which may help with the questions on the homework:
Ex 22.2 qu 1 (a, c, e)
Ex 22.3 qu 1 (a, c, e) and 2
In for Monday!
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Mr Brewin’s Year 9 – Sine and Cosine Rules7 comments to Mr Brewin’s Year 9 – Sine and Cosine Rules |
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Mr brewin, for questions e on both, they dont tell you the angle, and also for question 2. When using cosine, do you have to change the a^2=b^2+c^2 when the hyp has all ready been shown? its really confusing and i dont understand them, and on a question i got a negative answer and you cannot find the square root!!
Aalia, I don’t know which book you are using, but if it’s Sine and Cosine rules then the triangles won’t be right-angled so you shouldn’t be using Pythagoras.
Look at your lesson notes – there’s an extra term in the cosine rule that you mustn’t ignore.
They don’t tell you the angle because that is what the question wants you to find. The cos of an obtuse angle is negative so it’s OK to get a negative number. You shouldn’t, however, be trying to square-root it!
Have another go and bring your attempt to someone in the department for help.
JT
Come and see me tomorrow Aalia if you can.
i am completely stuck on ex22.3 1. e), how do you work it out? and also, for ex22.2 1. e), does 8 ÷ 10/sin61 = sin y ?
Sorry Sophie – not got the question sheet here with me, but what you’ve written for 22.2 1e sounds pretty good. If you could describe the question for 22.3 1 I’ll have a go, but sounds like you’ve put in a good stint, so just come see me tomorrow!
It tells you the measurements for all of the sides (6cm, 12cm and 8cm), but it doesn’t tell you any of the angles. You have to work out the angle opposite the 12cm side. I have no idea even where to begin!
OK, you have to rearrange the cosine rule to do that. If A is opposite the 12cm side:
A = Cos-1[ (6^2 +8^2 - 12^2)/(2 x 8 x 6) ]
Make sense?