cosx=±√[1-(sinx)^2] =±4/5
∵x为锐角
∴cosx=4/5
tanx=sinx/cosx=3/4
tan[x- (π/4)] = [tanx - tan(π/4)]/[1+tanxtan(π/4)] = (tanx-1)/(1+tanx) = (-1/4)/(7/4) =-1/7