高一数列的几道填空题等差数列an中a3+a7+2a15=40,则S19=?等差数列an中已知公差为1/2,且a1+a3+a5+.+a99=60,则S100=?求详细解答,在线等
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![高一数列的几道填空题等差数列an中a3+a7+2a15=40,则S19=?等差数列an中已知公差为1/2,且a1+a3+a5+.+a99=60,则S100=?求详细解答,在线等](/uploads/image/z/4088427-51-7.jpg?t=%E9%AB%98%E4%B8%80%E6%95%B0%E5%88%97%E7%9A%84%E5%87%A0%E9%81%93%E5%A1%AB%E7%A9%BA%E9%A2%98%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97an%E4%B8%ADa3%2Ba7%2B2a15%3D40%2C%E5%88%99S19%3D%3F%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97an%E4%B8%AD%E5%B7%B2%E7%9F%A5%E5%85%AC%E5%B7%AE%E4%B8%BA1%2F2%2C%E4%B8%94a1%2Ba3%2Ba5%2B.%2Ba99%3D60%2C%E5%88%99S100%3D%3F%E6%B1%82%E8%AF%A6%E7%BB%86%E8%A7%A3%E7%AD%94%2C%E5%9C%A8%E7%BA%BF%E7%AD%89)
高一数列的几道填空题等差数列an中a3+a7+2a15=40,则S19=?等差数列an中已知公差为1/2,且a1+a3+a5+.+a99=60,则S100=?求详细解答,在线等
高一数列的几道填空题
等差数列an中a3+a7+2a15=40,则S19=?
等差数列an中已知公差为1/2,且a1+a3+a5+.+a99=60,则S100=?
求详细解答,在线等
高一数列的几道填空题等差数列an中a3+a7+2a15=40,则S19=?等差数列an中已知公差为1/2,且a1+a3+a5+.+a99=60,则S100=?求详细解答,在线等
(1)a3+a7=2a5
2a5+2a15=4a10=40,所以,a10=10.
s19=19*(a1+a19)/2=190.
(2)a2+a4+~+a100=(a1+1/2)+(a3+1/2)~+(a99+1/2)=60+50*1/2=85
S100=a1+a2+...+a99+a100
=(a1+a3+a5+.+a99)+(a2+a4+~+a100)
=60+85=145
a3+a7=2a5
2a5+2a15=4a10=40,所以,a10=10.
s19=19*(a1+a19)/2=190.
a2+a4+~~~+a100=(a1+1/2)+(a3+1/2)~~~+(a99+1/2)=60+25=85
a3+a7=2a5
=>原式=2a5+2a15=40 a5+a15=20
∵a5+a15=a1+a19=a2+a18=……=a9+a11=2×a10
∴a10=10
S19=(a1+a19)+(a2+a18)+……+(a9+a11)+a10
=9×2a10+a10
=190
a2=a1+d=a1+1/2
a...
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a3+a7=2a5
=>原式=2a5+2a15=40 a5+a15=20
∵a5+a15=a1+a19=a2+a18=……=a9+a11=2×a10
∴a10=10
S19=(a1+a19)+(a2+a18)+……+(a9+a11)+a10
=9×2a10+a10
=190
a2=a1+d=a1+1/2
a4=a3+d=a3+1/2
……
a100=a99+1/2
共50项
S100=(a1+a3+a5+…+a99)+(a2+a4+a6…+a100)
=60+[(a1+a3+a5+…+a99)+50×1/2]
就是这样了
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