Sebuah deret geometri terdiri dari 8 suku. jumlah 3 suku pertama adalah 210 dan jumlah 3 suku terakhir adalah 6.720 suku pertama dari deret tersebut adalah
izharulhaq78
![a+ar+ar^2=210\\a(1+r+r^2)=210\\1+r+r^2=\frac{210}{a}\\\\ar^5+ar^6+ar^7=6720\\ar^5(1+ar+ar^2)=6720\\ar^5(\frac{210}{a})=6720\\210r^5=6720\\r^5=\frac{6720}{210}\\r^5=\frac{672}{21}\\r^5=32\\r=\sqrt[5]{32}\\r=2\\\\a(1+2+2^2)=210\\a(3+4)=210\\a(7)=210\\a=\frac{210}{7}\\\boxed{a=30}](https://tex.z-dn.net/?f=a%2Bar%2Bar%5E2%3D210%5C%5Ca%281%2Br%2Br%5E2%29%3D210%5C%5C1%2Br%2Br%5E2%3D%5Cfrac%7B210%7D%7Ba%7D%5C%5C%5C%5Car%5E5%2Bar%5E6%2Bar%5E7%3D6720%5C%5Car%5E5%281%2Bar%2Bar%5E2%29%3D6720%5C%5Car%5E5%28%5Cfrac%7B210%7D%7Ba%7D%29%3D6720%5C%5C210r%5E5%3D6720%5C%5Cr%5E5%3D%5Cfrac%7B6720%7D%7B210%7D%5C%5Cr%5E5%3D%5Cfrac%7B672%7D%7B21%7D%5C%5Cr%5E5%3D32%5C%5Cr%3D%5Csqrt%5B5%5D%7B32%7D%5C%5Cr%3D2%5C%5C%5C%5Ca%281%2B2%2B2%5E2%29%3D210%5C%5Ca%283%2B4%29%3D210%5C%5Ca%287%29%3D210%5C%5Ca%3D%5Cfrac%7B210%7D%7B7%7D%5C%5C%5Cboxed%7Ba%3D30%7D "a+ar+ar^2=210\\a(1+r+r^2)=210\\1+r+r^2=\frac{210}{a}\\\\ar^5+ar^6+ar^7=6720\\ar^5(1+ar+ar^2)=6720\\ar^5(\frac{210}{a})=6720\\210r^5=6720\\r^5=\frac{6720}{210}\\r^5=\frac{672}{21}\\r^5=32\\r=\sqrt[5]{32}\\r=2\\\\a(1+2+2^2)=210\\a(3+4)=210\\a(7)=210\\a=\frac{210}{7}\\\boxed{a=30}")
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