Different Answers using Cosine Rule vs Sine Rule

senseimichael

senseimichael

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Mar 11, 2020
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The model answer I worked out uses the Cosine Rule to find angle BAC. My student uses the Sine Rule, and her steps should be correct. But try as I might many times, both sets of answers are different! I even checked approximations and she has given approximations 1 s.f. higher in her intermediate answers. Anyone can tell me where I or she has gone wrong?

The Question:
1658069597784.png
My answer:
1658069639494.png
My student's answer (ignore her mistake with a^2, she definitely understood a=7.15420):
1658069565012.png
 
Without reading the details I'd bet on your student's answer because it is closer to 90 than 108, while yours is not.
 
senseimichael said:
The model answer I worked out uses the Cosine Rule to find angle BAC. My student uses the Sine Rule, and her steps should be correct. But try as I might many times, both sets of answers are different! I even checked approximations and she has given approximations 1 s.f. higher in her intermediate answers. Anyone can tell me where I or she has gone wrong?

The Question:
View attachment 33425
My answer:
View attachment 33426
My student's answer (ignore her mistake with a^2, she definitely understood a=7.15420):
View attachment 33424
Click to expand...
AB is NOT equal to 3.8.
 
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