2×2×(-1)+3×(-1)-1= -4-3-1=-8
...=а^2-7a-3a+21-6a^2+10a=-5a^2+0+21=-5a^2+21
Cos(x-5/(pi2))=cos(х) cos(5/(pi2))+sin(х) sin(5/(pi2))= sinx
<span>если у вас написано 5pi/2 то cos(5pi/2)=0 </span>
<span>sin(5pi/2)=1 </span>
<span>cos(2x)=1-2sin^2(x) </span>
<span>1-2sin^2(x)-sinx-1=0 </span>
<span>sinx(2sinx+1)=0 </span>
<span>sinx=0, </span>
<span>x= n pi </span>
<span>и </span>
<span>sinх= -0.5, </span>
<span>x= -pi/6+2pi n </span>
<span>x= -5pi/ 6+2pi n </span>
Всё подробно написала в решении..............................