1.cosa=a*b/|a|*|b|=7/5√2=7√2/10 2 x1/3
a2,a3+1 a4 q+q^3=2q^2+2 q=2 an=2^(n-1) bn={log2an}是以0为首项1为公差的等差数列
Sn=nb1+n(n-1)d
=n(n-1)/2
1.cosa=a*b/|a|*|b|=7/5√2=7√2/10 2 x1/3
a2,a3+1 a4 q+q^3=2q^2+2 q=2 an=2^(n-1) bn={log2an}是以0为首项1为公差的等差数列
Sn=nb1+n(n-1)d
=n(n-1)/2