15. (2023•烟台模拟)已知函数f(x)=cos(2x+φ)(0⩽φ<2π)f(x)=\cos (2x+\varphi )(0\leqslant \varphi <2\pi )f(x)=cos(2x+φ)(0⩽φ<2π)在[−π6,π4][-\frac{\pi }{6},\frac{\pi }{4}][−6π,4π]上单调递增,则φ\varphiφ的取值范围为((( )))