Question

sina*cosa=? если sina+cosa=1/3

Answer 1

$(sina+cosa)^2=3^{-2}\ 2sina*cosa=3^{-2}-1\ sina*cosa=frac{-4}{9}$

Answer 2

$(sin alpha +cos alpha )^2=sin^2 alpha +cos^2 alpha +2sin alpha cos alpha =1+2sin alpha cos alpha \ sin alpha cos alpha = cfrac{(sin alpha +cos alpha )^2-1}{2} \ sin alpha cos alpha = cfrac{( frac{1}{3} )^2-1}{2} =- cfrac{4}{9}$