Answer to Question #272663 in Differential Equations for Lando

Question #272663

Verify that the function 𝑦 = 𝑐1𝑒


(−𝑘+2𝑖)𝑥 + 𝑐2𝑒


(−𝑘−2𝑖)𝑥 is a


solution to


𝑦


′′ + 2𝑘𝑦


′ + (𝑘


2 + 4)𝑦 = 0


1
Expert's answer
2021-11-30T04:22:07-0500
y=c1e(k+2i)x+c2e(k2i)xy=c_1e^{(-k+2i)x}+c_2e^{(-k-2i)x}




y=(k+2i)c1e(k+2i)x+(k2i)c2e(k2i)xy'=(-k+2i)c_1e^{(-k+2i)x}+(-k-2i)c_2e^{(-k-2i)x}




y=(k+2i)2c1e(k+2i)x+(k2i)2c2e(k2i)xy''=(-k+2i)^2c_1e^{(-k+2i)x}+(-k-2i)^2c_2e^{(-k-2i)x}


Substitute


y+2𝑘y+(k2+4)𝑦y''+ 2𝑘y'+ (k^2+ 4)𝑦




=(k+2i)2c1e(k+2i)x+(k2i)2c2e(k2i)x=(-k+2i)^2c_1e^{(-k+2i)x}+(-k-2i)^2c_2e^{(-k-2i)x}


+2k(k+2i)c1e(k+2i)x+2k(k2i)c2e(k2i)x+2k(-k+2i)c_1e^{(-k+2i)x}+2k(-k-2i)c_2e^{(-k-2i)x}


+(k2+4)c1e(k+2i)x+(k2+4)c2e(k2i)x+ (k^2+ 4)c_1e^{(-k+2i)x}+ (k^2+ 4)c_2e^{(-k-2i)x}




=(k24ik42k2+4ik+k2+4)c1e(k+2i)x=(k^2-4ik-4-2k^2+4ik+k^2+4)c_1e^{(-k+2i)x}




+(k2+4ik42k24ik+k2+4)c2e(k2i)x+(k^2+4ik-4-2k^2-4ik+k^2+4)c_2e^{(-k-2i)x}




=0,xC=0, x\in \Complex

Therefore the function


y=c1e(k+2i)x+c2e(k2i)xy=c_1e^{(-k+2i)x}+c_2e^{(-k-2i)x}

is a solution to


y+2𝑘y+(k2+4)𝑦=0.y''+ 2𝑘y'+ (k^2+ 4)𝑦= 0.

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