令2x+φ=kπ+π/2
则x=kπ/2+π/4-φ/2
令kπ/2+π/4-φ/2=-π/8
则φ=kπ+3π/4
所以tanφ
=tan(kπ+3π/4)
=tan3π/4
=-tanπ/4
=-1