相似图形的性质题.1.已知 a/b = c/d (b±d≠0),求证: (a+c)/(a-c) = (b+d)/(b-d
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相似图形的性质题.
1.已知 a/b = c/d (b±d≠0),求证: (a+c)/(a-c) = (b+d)/(b-d)
2.已知: a/3 = b/5 = c/7 ,且3a+2b-4c = 9,求 1/a+1/b+1/c
1.已知 a/b = c/d (b±d≠0),求证: (a+c)/(a-c) = (b+d)/(b-d)
2.已知: a/3 = b/5 = c/7 ,且3a+2b-4c = 9,求 1/a+1/b+1/c
1)
证:(a+c)/(a-c) = (b+d)/(b-d)
(a+c)/(a-c) -(b+d)/(b-d)
=[(a+c)*(b-d)-(b+d)(a-c)]/[(a-c)*(b-d)]
=[ab-ad+bc-cd-(ab-bc+ad-cd)]/[(a-c)*(b-d)]
=2(bc-ad)/)[(a-c)*(b-d)]
因为a/b = c/d (b±d≠0),
得ad=bc
所以=2(bc-ad)/)[(a-c)*(b-d)]=0(分子为0了)
所以(a+c)/(a-c) = (b+d)/(b-d) 得证
2)a/3 = b/5 = c/7
设a=3k,b=5k,c=7k
因为3a+2b-4c = 9
所以9k+10k-28k=9
k=-1
a=-3,b=-5,c=-7
1/a+1/b+1/c=-1/3-1/5-1/7=-(35+21+15)/105
=-71/105
证:(a+c)/(a-c) = (b+d)/(b-d)
(a+c)/(a-c) -(b+d)/(b-d)
=[(a+c)*(b-d)-(b+d)(a-c)]/[(a-c)*(b-d)]
=[ab-ad+bc-cd-(ab-bc+ad-cd)]/[(a-c)*(b-d)]
=2(bc-ad)/)[(a-c)*(b-d)]
因为a/b = c/d (b±d≠0),
得ad=bc
所以=2(bc-ad)/)[(a-c)*(b-d)]=0(分子为0了)
所以(a+c)/(a-c) = (b+d)/(b-d) 得证
2)a/3 = b/5 = c/7
设a=3k,b=5k,c=7k
因为3a+2b-4c = 9
所以9k+10k-28k=9
k=-1
a=-3,b=-5,c=-7
1/a+1/b+1/c=-1/3-1/5-1/7=-(35+21+15)/105
=-71/105
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