Posted by **Matt** on Tuesday, November 6, 2012 at 10:31am.

Given that sinx = 7 /25 pi/2 <or equal to x <or equal to pi Find the value of cos 2x

- Trigonometric -
**Reiny**, Tuesday, November 6, 2012 at 10:38am
sinx = 7/25 , π/2 ≤ x ≤ π

so x is in quadrant II , making 2x fall in quadrant IV

So the opposite side is 7, and the hypotenuse is 25, so the adjacent side would be 24 by Pythagoras ,

and cosx = -24/25

cos 2x = cos^2 x - sin^2 x

= 576/625 - 49/625 = 527/625

- Trigonometric - a note -
**Steve**, Tuesday, November 6, 2012 at 11:22am
Just a note:

If π/2 ≤ x ≤ π, π ≤ 2x ≤ 2π which means 2x could be in QIII or QIV.

In this case, since x > 3π/4, 2x>3π/2, and is indeed in QIV, but it need not have been so.

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