Знайти значення виразу cos11°*cos21°+cos69°*cos79-cos10°

Cos11⁰·cos21⁰+cos69⁰·cos79⁰-cos10⁰=
=cos11⁰·cos21⁰+cos(90⁰-21⁰)·cos(90⁰-11⁰)-cos10⁰=
=cos11⁰·cos21⁰+sin21⁰·sin11⁰-cos10⁰=
=1/2[cos(11⁰-21⁰)+cos(11⁰+21⁰)]+1/2[cos(11⁰-21⁰)-cos(11⁰+21⁰)]-cos10⁰=
=cos(11⁰-21⁰)-cos10⁰=cos(-10⁰)-cos10⁰=cos10⁰-cos10⁰=0;