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    Hey this is from jan 06 paper. Got solutions but dont like the way they have done it and I know it can be done another way.

    OAB is a sector of a circle radius 5m. The chord AB is 6m long.

    a) Show that cos AOB = 7/25
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    It's just the cos rule- you have a triangle, sides of 5, 5 and 6 and you want one angle.
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    What solutions are you using?

    (Method 1)
    Cosine rule.

    (Method 2)
    Spot a 3-4-5 triangle and use the trig identity cos(2A) = 1 - 2sin^2(A).

    cos(AOB)
    = 1 - 2sin^2((1/2)AOB)
    = 1 - 2(3/5)^2
    = 1 - 18/25
    = 7/25
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    I cant get my head round this lol! I did this paper in jan and I know I got it right cause it was show that. I definitely used cos rule though
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    (Original post by Jonny W)
    What solutions are you using?

    (Method 1)
    Cosine rule.

    (Method 2)
    Spot a 3-4-5 triangle and use the trig identity cos(2A) = 1 - 2sin^2(A).

    cos(AOB)
    = 1 - 2sin^2((1/2)AOB)
    = 1 - 2(3/5)^2
    = 1 - 18/25
    = 7/25
    Method 1 would have been used. That trig identity is in C3, so presuming knowledge up to C2, candidate would not know to use this method.
 
 
 
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