a☆b=ab+b-a,則（b-a）☆b=(b-a)b+b-(b-a)=b²-ab+a
(x-2)(x+7.5)=x²+7.5x-2x-15=x²+5.5x-15
所以-k=5.5,即k=-5.5
a²b²+a²+b²+1=4ab
a²b²-2ab+1+a²+b²-2ab=0
(ab-1)²+(a-b)²=0
ab-1=0,a-b=0
解得a=b=1或a=b=-1
x²-y²=(x+y)(x-y)
所以2（x+y)=10,得x+y=5
a²-10a+1=0
a²+1=10a
a+1/a=10
(a+1/a)²=100
a²+1/a²+2=100
a²+1/a²=98
(a-1/a)²
=a²+1/a²-2
=98-2
=96
尾數(shù)是0
因?yàn)槠渲幸粋€因數(shù)2²+1=5,而其它的因數(shù)都是奇數(shù),所以最后尾數(shù)為5,減掉尾數(shù)為5的數(shù),結(jié)果尾數(shù)為0