sub in given values to get 2 equations.
[tex]a + b \cos(2*0) = -1 \rightarrow a+b = -1 \\ \\ a+b \cos(2*\frac{\pi}{2}) = 7 \rightarrow a - b = 7[/tex]
Solve system using elimination. Add the equations together, cancelling out 'b'
[tex]2a = 6 \\ \\ a = 3[/tex]
Sub into first equation to get value for 'b'
[tex]3+b = -1 \\ \\ b = -4[/tex]
