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cos de 15 grade ?cu explicatie

Răspuns :

Cos15=Cos(45-30)=Cos 45*cos30+sin45*sin30

15 grade=45 grade-30grade
cos15=cos(45-30)=cos45cos30+sin45sin30=√2/2x√3/2+√2/2x1/2=
=√6/4+√2/4=√2(√3+1)/4