当k为何值时，关于x的方程x2-（2k-1）x=-k2+2k+3，
（1）有两个不相等实数根？
（2）有两个相等实数根？
（3）没有实数根？
方程变形为x2-（2k-1）x+k2-2k-3=0，
（1）根据题意得△=（2k-1）2-4（k2-2k-3）＞0，
解得k＞-
，13 4
所以当k＞-
时，方程有两个不相等实数根；13 4
（2）根据题意得△=（2k-1）2-4（k2-2k-3）=0，
解得k=-
，13 4
所以当k=-
时，方程有两个相等实数根；13 4
（3）根据题意得△=（2k-1）2-4（k2-2k-3）＜0，
解得k＜-
，13 4
所以当k＜-
时，方程没有实数根．13 4
Which of your two hands do you use most? Very __ of us can use both of our hands equally（相等的） well. ___of us are right-handed. Only about five _out of a hundred are left-handed. New-born babies can hold things __either of their hands, but__ about two years they usually like to use their right hands. Scientists don’t know _ this happens.They _ that we inherited(遗传) this from our animalvancestors(祖先)but this may not be _ .Monkeys are our closest relatives(亲戚)in the animal _ . Scientists have found that monkeys prefer to use one of their hands more than ___ --,but it can be __ hand. There are as many right-handed monkeys_ _ left-handed ones . Next time you visit the zoo, ___ the monkeys carefully. You’ll see that some of them will use their left hands. But most human beings use their right hands _ and this __ life difficult for the left-handed people. We live in a right-handed world.
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