[tex]\frac{1}{cos^{2}15} + \frac{1}{sin^{2}15} =\frac{sin^{2}15+cos^{2}15}{sin^{2}15cos^{2}15}=\frac{1}{cos^{2}15sin^2{15}}=\frac{4}{4sin^2{15}cos^2{15}}=\\ =\frac{4}{sin^{2}30}=4\cdot4=16[/tex]
Explicație pas cu pas:
16
