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