$\text{a) CE = BC – BE = 25 – 9 =16 = CD}$
$\triangle$$ABE_{}$ $\text{cân tại B =>}$ $\widehat{BAE}$ $\text{=}$ $\widehat{BEA}$
$\triangle$$CED_{}$ $\text{cân tại C =>}$ $\widehat{CED}$ $\text{=}$ $\widehat{CDE}$
$\text{=>}$ $\widehat{BEA}$ $\text{+}$ $\widehat{CED}$
$\text{=}$ $\widehat{BAE}$ $\text{+}$ $\widehat{CDE}$
$\text{=}$ $90^0$ $-_{}$ $\widehat{EAD}$ $\text{+}$ $90^0$ $-_{}$ $\widehat{ADE}$
$\text{=}$ $180^0$ $-_{}$ ($\widehat{EAD}$ $\text{+}$ $\widehat{ADE}$)
$\text{=}$ $180^0$ $-_{}$ ($180^0$ $-_{}$ $\widehat{AED}$)
$\text{=}$ $\widehat{AED}$
$\text{=>}$ $\widehat{BEA}$ $\text{+}$ $\widehat{CED}$ $\text{=}$ $\widehat{AED}$
$\text{Mà}$ $\widehat{BEA}$ $\text{+}$ $\widehat{CED}$ $\text{+}$ $\widehat{AED}$ $\text{=}$ $180^0$
$\text{=>}$ $\widehat{BEA}$ $\text{+}$ $\widehat{CED}$ $\text{=}$ $\widehat{AED}$ $\text{=}$ $90^0$
