Cos105°+cos75 объясните как решить.

Cos(105) = cos (60+45)
cos (75) = cos (30+45)
cos (60+45) = 1/2×√2/2 –√3/2×√2/2
√2/2(1/2–√3/2) =(1–√3)/2√2
cos(30+45) = √3/2×√2/2 –1/2×√2/2
√2/2(√3/2-1/2) = (√3–1)/2×√2/2= (√3–1)/2√2
(1–√3) – (1–√3) = 0