f(x)=asinx+bcosx=√a²+b²sin(x+φ)
a²+b²=8
f(π/4)=√2/2(a+b)=-2√2 or 2√2,
a+b=-4 or 4
∴a=b=-2 or a=b=2
g(x)=2sinx+2 or g(x)=-2sinx-2