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2cos^2 (пи+а) / 4-2sin^2 (пи+а) / 4

= 2 сos^2 (pi/4 + a/4) - 2 sin^2 (pi/4 + a/4) =

=2 (cospi/4*cos a/4-sin pi/4 * sin a/4) ^2 - 2 (sin pi/4 * cos a/4+cos pi/4*sin a/4) ^ =

= 2 (sgrt 2/2 * cos a/4-sgrt2/2*sin a/4) ^2 - 2 (sgrt2/2 * cos a/4 + sgrt2/2*sin a/4) ^2=

=2 * (sgrt2/2) ^2 (cos a/4 - sin a/4) ^2 - 2 * (sgrt2/2) ^2 * (cos a/4 + sin a/4) ^2 =

= 2*2/4 (cos^2 (a/4) - 2sin a/4*cos a/4 + sin^2 (a/4) -

- 2*2/4 (cos^2 (a/4) + 2 sin a/4 cos a/4 + cos^2 (a/4) =

= (1 - 2 sin (a/4) cos (a/4)) - (1 + 2 sin (a/4) cos (a/4) =

=1 - sin (a/2) - 1 - sin (a/2) = - 2 sin (a/2)