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;
7a^3^b^3^-77a^2^b^3^-21a^3^b^4^=7a^2^b^3^(a-11-3ab); 5a^3^x^2^y^2^-15a^3^xy^2^-5a^4^y=5a^3^xy(xy-3y-a^3^
Sn = (a1+ an)*n/2
S50 = (-3 + 43) * 50/2 = 1000.