E(cosx)=[-1;1], =>
-1≤a²-a-1≤1
1. a²-a-1≥-1, a²-a≥0. a*(a-1)≥0. a*(a-1)=0. a₁=0, a₂=1
++++[0]-----[1]++++>a
a≤0, a≥1
2. a²-a-1≤1, a²-a-2≤0. a²-a-2=0. a₁=-1, a₂=2
+++++[-1]-----[2]++++>a
a∈[-1;2]
3.
\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \
--------[-1]------[0]--------[1]------[2]--------->a
/ / / / / / / / / / / / / / / / / /
4. a∈[-1;0]∪[1;2]