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问题 解答题

在△ABC中，已知（a2+b2）sin（A-B）=（a2-b2）sin（A+B）

证明：△ABC是等腰三角形或直角三角形．

答案

证：∵（a2+b2）sin（A-B）=（a2-b2）sin（A+B），

∴（a2+b2）（sinAcosB-cosAsinB）=（a2-b2）（sinAcosB+cosAsinB），

化简整理得：a2cosAsinB=b2sinAcosB，

由正弦定理得sin2AcosAsinB=sin2BsinAcosB，

即sinAcosA=sinBcosB，即sin2A=sin2B，

∴A=B或A+B=

π
2
，

则△ABC是直角的三角形或等腰三角形．

单项选择题

根据有关规定，单只基金持有的全部权证市值占基金资产净值的比例不得超过( )。

A．0．5%

B．1%

C．3%

D．5%
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单项选择题

Psychological maladjustment in children is caused by the stress of the birthing process as is proved by the discovery of a positive relationship between the duration of the mother’s labor and the amount of time the child spent crying in the first month of life. Which of the following, if true, LEAST damages the author’s assertion

A．There is no relationship between the amount of time spent crying and psychological maladjustment.

B．Behavior indicative of psychological maladjustment does not appear until the third month of a child’s life.

C．From the infant’s point of view, a hurried labor is more stressful than a gradual, slow delivery.

D．The estimates of the duration of labor were based on obstetricians’ estimates of the time of the onset of labor.

E．(E) The infants who have experienced the greatest stress during birth are often too weak to cry for extended periods of time.
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