$50R_{1}-$ $R_{1}^{2}=625$
⇔ $0 =$ $R_{1}^{2}$ - $50R_{1}+625$
⇔ $R_{1}^{2}-$ $50R_{1}+625$ $=0$
⇔ $R_{1}^{2}$ $-25R_{1}$ $-25R_{1}$ $+625$$=0$
⇔ $R_{1}$ ($R_{1}-25$) $-25$($R_{1}-25$) $=0$
⇔ $(R_{1}-25).$ $(R_{1}-25)=$ $=0$
⇔ $(R_{1}-25) ^{2}$ $=0$
⇔ $(R_{1}-25)$ $=0$
⇔ $R_{1}=25$
$chucbanhoktot$
$chomikxinctlhnnha$