Toán Lớp 11: tính
5+15+45+135+⋯+32805+98415
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Toán Lớp 11: tính
5+15+45+135+⋯+32805+98415
Comments ( 1 )
=5 + 5.3 + 5.3^2 +5.3^3 +…+ 5.3^9
=5( 1 + 3 + 3^2 + 3^3 +…+ 3^9)
text{Gọi:}A = 1 + 3 + 3^2 + 3^3 +…+ 3^9
=> 3A = 3 + 3^2 + 3^3 +…+ 3^10
=> A = \frac{(3A – A)}{2} = \frac{(3^10 – 1)}{2} = 29524
=> 5+15+45+135+⋯+32805+98415 = 5. 29524= 147620