Practicing Success
Solution of differential equation $\frac{dy}{dx}=cos(x+y+3)$ is : |
$y=-sin(x+y+3) + c$ $y=sin(x+y+3) + c$ $y=2tan^{-1}(x+c)-x-3$ $y=\frac{1}{2}tan^{-1}(x+c)+x+3$ |
$y=2tan^{-1}(x+c)-x-3$ |
The correct answer is Option (3) → $y=2tan^{-1}(x+c)-x-3$ $\frac{dy}{dx}=\cos(x+y+3)$ let $z=x+y+3$ so $\frac{dz}{dx}=1+\frac{dy}{dx}$ $\frac{dz}{dx}-1=\cos z$ so $\int\frac{1}{1+\cos^2z}dz=\int dx$ $=\int\frac{1-\cos z}{1+\cos^2z}dz=\int dx$ $=\int\frac{1}{\sin^2z}-\frac{\cos z}{\sin^2z}dz=\int dx$ $⇒\int cosec^z-cosec\,z\cot z\,dz=\int dz$ $≡cosec\,z-\cot z=x+c$ $\frac{1-\cos z}{\sin z}=x+c$ $=\frac{2\sin^2\frac{z}{2}}{2\sin\frac{z}{2}\cos\frac{z}{2}}=x+c$ $\tan\frac{z}{2}=x+c$ $z=2\tan^{-1}(x+c)$ $x+y+3=2\tan^{-1}(x+c)$ $y=2\tan^{-1}(x+c)-x-3$ |