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# How to rearrange this equation to find x? watch

1. Okay, so I'm doing the cosine rule in maths and I can't figure out how to rearrange this ugly formula to get the length of one of the sides of a triangle. I've inputted all of the numbers but am struggling with the rearranging. Could someone please go over it with me step by step?

9.92 = x2+7.12 + x(14.2cos(47))

I then minused the 7.1 squared to get this:

9.92 - 7.12 = x2 + x(14.2cos(47))

then i totalled the left hand side to get this:

47.6 = x2 + x(14.2cos(47))

but it is still really disgusting and I can't figure it out help?
2. (Original post by Bhansongal98)
Okay, so I'm doing the cosine rule in maths and I can't figure out how to rearrange this ugly formula to get the length of one of the sides of a triangle. I've inputted all of the numbers but am struggling with the rearranging. Could someone please go over it with me step by step?

9.92 = x2+7.12 + x(14.2cos(47))

I then minused the 7.1 squared to get this:

9.92 - 7.12 = x2 + x(14.2cos(47))

then i totalled the left hand side to get this:

47.6 = x2 + x(14.2cos(47))

but it is still really disgusting and I can't figure it out help?
Assuming you were right to use the cos rule here (I suspect you might have chosen the wrong sides to substitute), you could complete the square. Evaluate cos47.
3. (Original post by Kvothe the arcane)
Assuming you were right to use the cos rule here (I suspect you might have chosen the wrong sides to substitute), you could complete the square. Evaluate cos47.
I have got A(47o) , c(7.1cm) and a(9.9cm) and I'm trying to work out b(x)

I've put cos47 in the calc and come out with 0.6819983601

The homework is on the cosine rule so I'd be worried if it wasn't
4. (Original post by Bhansongal98)
I have got A(47o) , c(7.1cm) and a(9.9cm) and I'm trying to work out b(x)

I've put cos47 in the calc and come out with 0.6819983601

The homework is on the cosine rule so I'd be worried if it wasn't
Any chance you can post a diagram?

I would have worked out angle B using the sin rule.

And then said that

Now that you have 0.682, you can substitute it into your equation and complete the square (or use quadratic formula) to get an answer for x. You can disregard the negative solution.

5. (Original post by Kvothe the arcane)
Any chance you can post a diagram?

I would have worked out angle B using the sin rule.

And then said that

Now that you have 0.682, you can substitute it into your equation and complete the square to get an answer for x. You can disregard the negative solution.
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6. (Original post by Kvothe the arcane)
Any chance you can post a diagram?

I would have worked out angle B using the sin rule.

And then said that

Now that you have 0.682, you can substitute it into your equation and complete the square (or use quadratic formula) to get an answer for x. You can disregard the negative solution.

I got 3.6 to 1dp? does this sound right to you? I completed the square like you said
7. (Original post by Bhansongal98)
I got 3.6 to 1dp? does this sound right to you? I completed the square like you said
I think you had a - error so you got the wrong solution
8. (Original post by Kvothe the arcane)
I think you had a - error so you got the wrong solution
9. (Original post by Bhansongal98)
9.92 - 7.12 = x2 + x(14.2cos(47))
10. (Original post by Kvothe the arcane)
9.92 - 7.12 = x2 + x(14.2cos(47))

Ah cool, I mixed my signs then? I've got to go back to class now but thank you kindly for your help!
11. (Original post by Bhansongal98)
Ah cool, I mixed my signs then? I've got to go back to class now but thank you kindly for your help!
Yup. No worries. I'm glad to be of assistance .

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