∵a2-1=（a+1）（a-1），
∴a2-1a-1=(a-1)(a+1)a-1=a+1；
当a=99时，a+1=100．
Jim is eight-year-old boy. He spends half _____hour playing _____ piano every day.
A．a; an; a
B．an; the; an
C．an; an; the
D．the; a; /
两个12V的蓄电池并联到一起后的电压是多少（）。
A.6.0
B.12.0
C.24.0
D.0.0
