已知{An}是等差数列,A1+A2=4,A7+A8=28,则该数列前十项和S10=?
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![已知{An}是等差数列,A1+A2=4,A7+A8=28,则该数列前十项和S10=?](/uploads/image/z/8827888-40-8.jpg?t=%E5%B7%B2%E7%9F%A5%EF%BD%9BAn%EF%BD%9D%E6%98%AF%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%2CA1%2BA2%EF%BC%9D4%2CA7%EF%BC%8BA8%EF%BC%9D28%2C%E5%88%99%E8%AF%A5%E6%95%B0%E5%88%97%E5%89%8D%E5%8D%81%E9%A1%B9%E5%92%8CS10%EF%BC%9D%3F)
已知{An}是等差数列,A1+A2=4,A7+A8=28,则该数列前十项和S10=?
已知{An}是等差数列,A1+A2=4,A7+A8=28,则该数列前十项和S10=?
已知{An}是等差数列,A1+A2=4,A7+A8=28,则该数列前十项和S10=?
A7+A8
=A1+6d+A2+6d
=A1+A2+12d
=4+12d=28
d=2
A1+A2=2A1+d=4
A1=1
S10=10A1+10x9xd/2
=10+10x9x2/2
=10+90
=100