$\sin {25}^o-\cos {155}^o\cdot \mathrm{tg}{155}^o=\sin {25}^o-\cos {155}^o\cdot \frac{\sin {155}^o}{\cos {155}^o=}\sin {25}^o-\sin {155}^o=\sin {25}^o-\sin \left({180}^o-{25}^o\right)=\sin {25}^o-\sin {25}^o=0$