помогите по братски

√72-√288*sin²(21π/8)=√72-√(4*72)*sin²(21π/8)=√72-2√72*sin²(21π/8)=√72*(1-2sin²(21π/8))=√72*cos(2*(21π/8))=√72cos(21π/4)=√72cos(5π+π/4)=√72*(-cos(π/4))=-√72*сos(π/4)=-√72*(√2/2)=-√144/2=-12/2=-6
ответ: -6