Вот..................................
Cosa=-0,8
sin²a=1-cos²a,sin²a=1-0,64=0,36,/sina/=√0,36, /sina/=0,6
cos2a=cos²a-sin²a=0,64-0,36=0,28
3cos2a=3.0,28
3cos2a=0,84
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У'=9-х^2
9-х^2=0
х=+-3
у(-3)=5-27+9=-13
у(3)=5+27-9=23
Ответ:23