7,3,-1
a1=7
a2=3
a2-a1=3-7=-4
d=-4
a4=a1+3d=7+3*-4=-5
a5=a1+4*d=7+4*-4=-9
S5=2a1+d(n-1)/2*n=14-4*4/2*5=-2/2*5=-5
Cos²x+2sinx+2=0
1-sin²x+2sinx+2=0
sinx=a
a²-2a-3=0
a1+a2=2 U a1*a2=-3
a1=-1⇒sinx=-1⇒x=-π/2+2πn
-4π≤-π/2+2πn≤2π
-8≤-1+4n≤4
-7≤4n≤5
-7/4≤n≤5/4
n=-1⇒x=-π/2-2π=-5π/2
n=0πx=-π/2
n=1πx=-π/2+2π=3π/2
a2=3⇒sinx=3>1 нет решения