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Question 4: Answer 5, the compulsory course of senior two mathematics.
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3sinA = 5sinB (this is known)

According to sine theorem

∴3a=5b,b=3/5a

b+c=2a

∴3/5a+c=2a

∴c=7/5a

According to cosine theorem:

cosC=(a? +b? -c? )/(2ab)

=(a? +9/25a? -49/25a? )/(2*a*3/5a)

=(- 15)/30

=- 1/2

∴C= 120?

Option b

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