# Double angle formulae

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#1
For 16b in the answer it says 2 theta is acute, can someone explain why its acute
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#2
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3 weeks ago
#3
well since the cosine is positive the angle cannot be obtuse....
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#4
(Original post by the bear)
well since the cosine is positive the angle cannot be obtuse....
Can I say that cos to the -1 (5/9) then divide by 2 = 28.1 therefore theta is acute
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3 weeks ago
#5
(Original post by Jshek)
Can I say that cos to the -1 (5/9) then divide by 2 = 28.1 therefore theta is acute
in a triangle it is possible for an angle to be acute or obtuse. if it were obtuse the cosine would be negative.
since in this case the cosine is positive we can say right away that the angle is acute.
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#6
(Original post by the bear)
in a triangle it is possible for an angle to be acute or obtuse. if it were obtuse the cosine would be negative.
since in this case the cosine is positive we can say right away that the angle is acute.
If cos is positive, how do you know angle is acute? Do u have to draw cast diagram
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3 weeks ago
#7
(Original post by Jshek)
If cos is positive, how do you know angle is acute? Do u have to draw cast diagram
i have a mental image of the cos graph...

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