令x=sina y=1+cosa
x+y+c
=sina+1+cosa+c
=√2sin(a+π/4)+c+1≥0
√2sin(a+π/4)≥-c-1
sin(a+π/4)≥-1
√2sin(a+π/4)≥-√2
当-c-1≤-√2时,不等式恒成立.
c≥√2-1
c的取值范围为[√2-1,+∞)
令x=sina y=1+cosa
x+y+c
=sina+1+cosa+c
=√2sin(a+π/4)+c+1≥0
√2sin(a+π/4)≥-c-1
sin(a+π/4)≥-1
√2sin(a+π/4)≥-√2
当-c-1≤-√2时,不等式恒成立.
c≥√2-1
c的取值范围为[√2-1,+∞)