20.
已知:如图,在△ABC中,点D、E分别在边BC、AB上,BD=AD=AC,AD与CE相交于点F,AE2=EF•EC.
(1)求证:∠ADC=∠DCE+∠EAF;
(2)求证:AF•AD=AB•EF.
(1)求证:∠ADC=∠DCE+∠EAF;
(2)求证:AF•AD=AB•EF.
18.化简:$\frac{{x}^{2}-x-6}{{x}^{2}-4}$÷$\frac{{x}^{2}-6x+9}{{x}^{2}-2x}$,并求当x=${3}^{\frac{1}{2}}$时的值.
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