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2014-09-19T19:18:11+03:00
1) sin75= sin(30+45)=sin30cos45 + sin45cos30=  \frac{1}{2} · \frac{ \sqrt{2} }{2}  \frac{ \sqrt{2} }{2} · \frac{ \sqrt{3} }{2}  \frac{ \sqrt{2} }{4} + \frac{ \sqrt{6} }{4} = \frac{ \sqrt{2}+ \sqrt{6}  }{4}
2) cos15=cos(45-30)=cos45cos30+sin45sin30= \frac{ \sqrt{2} }{2} · \frac{ \sqrt{3} }{2}  \frac{ \sqrt{2} }{2} · \frac{1}{2}  \frac{ \sqrt{6} }{4} + \frac{ \sqrt{2} }{4} = acelasi rezultat ca la 1
3) α∈ C₃
sin²α+cos²α=1
sin²α+(  \frac{-5}{13} )²=1
sin²α+  \frac{25}{169} =1
sin²α=  \frac{144}{169}
sinα= + si -  \frac{12}{13}
α∈C₃⇒ sinα = -  \frac{12}{13}
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