I missed class last Thursday and I am completely confused about the homework. I tried reading the text but there aren't any example problems like this one. Anyone know any free resources for James Stewart Single Variable Calculus? Well here is the question: For what values of k does the function y=cos kt satisfy the differential equation 4y"=-25y. For those values of k, verify that every member of the family of functions y=A sin kt + B cos Kt. I tried solving for y" and setting the equations equal to eachother. To tell the truth I don't understand what the question is saying. Thanks for any help!