guoguo2005 幼苗
共回答了89个问题 举报
ab=[(a+b)^2-(a-b)^2]/4
=(13^2-3^2)/4
=40
也可以先求出a=(13+3)/2=8,b=8-3=5或b=13-b=5,再求ab=8*5=40
=(13^2-3^2)/4
=40
也可以先求出a=(13+3)/2=8,b=8-3=5或b=13-b=5,再求ab=8*5=40
1年前
2
沉静之美 幼苗
共回答了5个问题 举报
(a-b)^2=a^2-2ab+b^2=3^2=9
(a+b)^2=a^2+2ab+b^2=13^2=169
则：(a+b)^2-(a-b)^2=4ab=160
所以ab=40
（如果还没懂可以再问,随时欢迎！）
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(a+b)^2=a^2+2ab+b^2=13^2=169
则：(a+b)^2-(a-b)^2=4ab=160
所以ab=40
（如果还没懂可以再问,随时欢迎！）
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ab=[(a+b)^2-(a-b)^2]/4<br/> =(13^2-3^2)/4<br/> =40<br/>也可以先求出a=(13+3)/2=8,b=8-3=5或b=13-b=5,再求ab=8*5=40 </div>
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<span>1年前</span>
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解 a-b=3 a+b=13 可得 a=8 b= 5 <br/>所以 ab=8*5=40 </div>
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<span>1年前</span>
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∵(a+b)²-（a-b)²<br/>=a²+2ab+b²-(a²-2ab+b²)<br/>=a²+2ab+b²-a²+2ab-b²<br/>=4ab<br/>∴4ab=(a+b)²-(a-b)²<br/>∴ab=[(a+b)²-(a-b)²]/4<br/>∴ab=(13²-3²)/4=(169-9)/4=160/4=40 </div>
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<span>1年前</span>
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a－b＋a＋b＝3＋13＝2a＝16<br/>所以a＝8 又因为a－b＝3<br/>所以b＝5 所以ab＝40<br/>我用手机答的，很辛苦的，分给我吧～ </div>
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<span>1年前</span>
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a+b=13<br/>(a+b)^2=169=a^2+2ab+b^2<br/>(a-b)^2=9=a^2-2ab+b^2<br/>两个式子相减：160=4ab<br/>ab=40 </div>
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<span>1年前</span>
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a-b=3 (1)<br/>a+b=13 (2)<br/>(1)+(2)得2a=16所以a=8,b=5 ，ab=8*5=40 </div>
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<span>1年前</span>
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