Science:Math Exam Resources/Courses/MATH307/December 2010/Question 06 (c)/Hint 1

Use Parseval's identity

$\sum _{n=-\infty }^{\infty }|c_{n}|^{2}={\frac {1}{L}}\int _{0}^{L}|f(x)|^{2}\,dx,$

and your results from part (a) and part (b).