I am having trouble with this question in physics:

 

An acoustic signal is composed of the first three harmonics of a wave of fundamental frequency 300 Hz. If these harmonics are described, in order, by cosine waves with amplitudes of 0.210, 0.340, and 0.170, what is the total amplitude of the signal at time 0.689 seconds? Assume the waves have phase angles θ_n = 0.

 

I understand that I should use the formula (sorry is formatting is not correct) F(t)=A1cos(2pif1t-THETAn)+A2cos(4pif1t-THETAn)+......

 

I have subbed in 0.210 for A1, 0.340 for A2, 0.170 for A3, 300 Hz for f1, and 0.689s for t to get an answer of -0.0149. When I put in the answer is says it is wrong. Any help on where I am going wrong??

 

Check your maths - I don't get that answer for

[latex]\sum_{k=1}^{k=3}A_k\cdot \cos (2\pi\cdot k\cdot f_0\cdot t)[/latex]