sina-cosa=17/13 возведем в квадрат
(sina-cosa)²=(17/13)²
sin²a-2sinacosa+cos²a=289/169 ; 2sinacosa=sin2a; sin²a+cos²a=1
1-sin2a=289/169
sin2a=1-289/169=(169-289)/169=-120/169
169sin2a=169(-120/169)=-120
sin a - cos a =17/13
( Sin a - cos a)^2=(17/13)^2
sin^2a-2 sincos a+cos^2=289/169
2 sin a cos a = sin^2a
sin^2a+ cos^2 a=1
1-sin2a=289/169
169 sin ^2a=169(-120/169)=-120
