1){x| x=2kπ+3π/4,or 2kπ+π/4, k ∈Z}
2){x|x=2kπ-arcsin(1/4) or x=(2k+1)π+arcsin(1/4),k ∈Z}
3){x|x=(2k+1)π±π/6,k ∈Z}