Can anyone please explain to me what exactly was done in the encircled two lines and why? I've tried figuring it out but I just can't make any sense of it.
Thank you for your time. Photo

Can anyone please explain to me what exactly was done in the encircled two lines and why? I've tried figuring it out but I just can't make any sense of it.
Thank you for your time. Photo

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I think you have added the two equations: \begin{align}c_2^{'}(x)\cos^2x&=\sin x \cos^2x \tag{1}\\ c_2^{'}(x)\sin^2x&=0 \tag{2} \end{align} Now, add them; $$c_2^{'}(x)[\cos^2x+\sin^2x]=\sin x \cos^2x.$$ Since, $\cos^2x+\sin^2x=1$, you get $c_2^{'}(x)=\sin x \cos^2x$.