Question

Nilai

Answer 1

Gunakan identity :
cos(x) = sin(90 - x) dan
sin^2 (x) + cos^2 (x) = 1

perhatikan bahwa :
cos^2 (15) = sin^2 (90-15) = sin^2 (75)
cos^2 (35) = sin^2 (90-35) = sin^2 (55)
so,
cos^2 (15) + cos^2 (35) + cos^2 (55) + cos^2 (75)
= sin^2 (75) + sin^2 (55) + cos^2 (55) + cos^2 (75)
= 1 + 1
= 2