1.1-2+2^2-2^3+...+2^8=?2.等差数列{an}中若a_4 a_7+a_5 a_6=4,则a1 a2

1 原式=[1-（-2）^8]/[1-(-2)]
=(1-256)/3
=-85
2 (1)
a2+a3+a6+a9+a10=20
a1+d+a1+2d+a1+5d+a1+8d+a1+9d=20
5a1+25d=20 a1+5d=4
a5+a7=a1+4d+a1+6d=2a1+10d=8
(2) a8+a9=a7+a10=a6+a11=a5+a12=a4+a13=a3+a14=a2+a15=a1+a16
=64/8=8
(3) a5=a1+4d=20
a25=a1+24d=-20
d=-2