I have two problems I need help with please.
1. find the exact value of tan(2u) using double angle forumlas given tan(u)= -2/9, pi/2<u<pi
2. use sum-to-product formulas to write the sum as a product?
sin(5theta) - sin(theta)
$\displaystyle \tan(2u) = \frac{2\tan{u}}{1 - \tan^2{u}}$
substitute (-2/9) for tan(u) and do the arithmetic.
$\displaystyle \sin{a} - \sin{b} = 2\cos\left(\frac{a+b}{2}\right)\sin\left(\frac{a-b}{2}\right)
$
substitute $\displaystyle 5\theta$ for a and $\displaystyle \theta$ for b.