Câu 1.

 \(\dfrac{R_1}{R_2}=r:\dfrac{r}{2}=2\)

Mà \(\dfrac{l_1}{l_2}=\dfrac{R_1}{R_2}=2\Rightarrow l_2=20:2=10cm\)

\(\dfrac{a}{c}=\dfrac{a^2+b^2}{b^2+c^2}\)

\(VP=\dfrac{a^2+ac}{ac+c^2}=\dfrac{a\left(a+c\right)}{c\left(a+c\right)}=\dfrac{a}{c}=VT\left(đpcm\right)\)

Chọn D.