If the characteristic equation has complex roots of the form r 1 = a + b i {\displaystyle r_{1}=a+bi} and r 2 = a − b i {\displaystyle r_{2}=a-bi} , then the general solution is accordingly y ( x ) = c 1 e ( a + b i ) x + c 2 e ( a − b i ) x {\displaystyle y(x)=c_{1}e^{(a+bi)x}+c_{2}e^{(a-bi)x}\,} . However, by Euler's formula , which states that e i θ = cos θ + i sin θ {\displaystyle e^{i\theta }=\cos \theta +i\sin \theta \,} , this solution can be rewritten as follows: