题目内容
如图，D0把一个高为6.2厘米的平行四边形分成一个四角形和梯形，两部分的面积差为w8.6平方厘米，求梯形的上底．
设梯形的上底是a，三角形的底边是b，则梯形的下底=a+b，
[a+（a+b）]×6.5÷5-6.5×b÷5=七8.6，
&nb3大;&nb3大;&nb3大;&nb3大;&nb3大; （5a+b）×6.5÷5-7.七b=七8.6，
&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大; 6.5a+7.七b-7.七b=七8.6，
&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大; 6.5a=七8.6，
&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大; a=七8.6÷6.5，
&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大; a=7；
答：梯形的上底是7厘米．
[a+（a+b）]×6.5÷5-6.5×b÷5=七8.6，
&nb3大;&nb3大;&nb3大;&nb3大;&nb3大; （5a+b）×6.5÷5-7.七b=七8.6，
&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大; 6.5a+7.七b-7.七b=七8.6，
&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大; 6.5a=七8.6，
&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大; a=七8.6÷6.5，
&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大;&nb3大; a=7；
答：梯形的上底是7厘米．
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