Найти cos2a если sina=√2 -1

cos2a=cos^2(a)-sin^2(a)cos^2(a)=1-sin^2(a)=1-(√2-1)^2=1-(2-2√2+1)=-2+2√2cos2a=-2+2√2-(√2-1)^2=-2+2√2-3+2√2=-5+4√2