Find the equation of a sine function with its first positive maximum point at (pi/5,8) and its next minimum point at (7pi/10,-2). With the sin form as y=a*sin[b*(x-c)] + d I found: a=[max-min]/2 = 5 b= 2pi/period= 2pi/(2*[max-min])=2 d=max+min/2=3 I am having troubles determining the value for c. How I am thinking about is that the point of reference for sin when it hits its first maximum is at pi/2. The difference between the pi/2 and pi/5 would be my phase shift however the answer is not right. The value for c should be pi/20. Can someone explain what I have done incorrectly here?