# Trig Help

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Given that cos2x=7/12 and "270 equal or < 2x equal or < 360", find sinx.

Please help and Thank you

• Trig Help -

In these type of quesstions it usually asks for the "exact" value of ....

cos 2x = 7/12

cos 2x = 1 - 2sin^2 x
2 sin^2 x = 1 - cos 2x = 1 - 7/12 = 5/12
sin^2 x = 5/24
sin x = ± √(5/24)
but 270 < 2x ≤ 360
135 ≤ x ≤ 180 ---> x in in I or II, so
sinx = +√5/√24 = √30/12

• Trig Help -

how did you get the ã30/12?

• Trig Help -

sqrt(5)/2sqrt(6)

when you rationalize the denominator you times both top and bottom by sqrt(6) only not the 2 right?

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