$證明:(2)如圖,$
$過(guò)點(diǎn)C作CG⊥AB,交AB的延長(zhǎng)線于點(diǎn)G,$
$過(guò)點(diǎn)F作 FH⊥DE,交DE的延長(zhǎng)線于點(diǎn)H$
$∵∠ABC=∠DEF，且∠ABC,∠DEF都是鈍角$
$∴180°-∠ABC=180°- ∠DEF$
$即 ∠CBG = ∠FEH$
$在△CBG 和 △FEH 中$
$\begin{cases}{ ∠G=∠H }\ \\ { ∠CBG=∠FEH } \\{ BC=EF} \end{cases}$
$∴△CBG≌△FEH(\mathrm {AAS})$
$∴CG=FH$
$在Rt△ACG和Rt△DFH中$
${{\begin{cases} {{AC=DF}} \\ {CG=FH} \end{cases}}}$
$∴Rt△ACG≌Rt△DFH(\mathrm {HL})\ $
$∴∠A=∠D$
$在△ABC和△DEF中$
${{\begin{cases} {{∠ABC=∠DEF}} \\ {∠A=∠D} \\ {AC=DF} \end{cases}}}$
$∴△ABC≌△DEF(\mathrm {AAS})$
