A = 71 + 72 + 73 + 74 + ...............................+ 7 2019
= 7 ( 1+2+3+4+..............+2019)
=> 1+2+3+4...........+2019)
= (1+2019) x 2019 :2
= 2039190
=> A = 7(2039190)
câu b
B = 101 + 102 +103 + ................................+102019
B = 10 (1+2+3+...................+2019)
=> (1+2+3+................+2019)
= (1+2019) x 2019 : 2
= 2039190
=> B = 10 2039190
Tần Nguyễn Phương Vy sai rồi. Mất căn bản nhá.
101+102 = 110
mà 10(1+2) lại bằng 1000
Sai hết rồi nhá
Giải:
\(A=7^1+7^2+7^3+7^4+...+7^{2019}\)
\(7A=7^1.7+7^2.7+7^3.7+7^4.7+...+7^{2019}.7\)
\(7A=7^2+7^3+7^4+...+7^{2019}+7^{2020}\)
\(7A-A=6A=\left(7^2+7^3+7^4+...+7^{2019}+7^{2020}\right)-\left(7^1+7^2+7^3+...+7^{2018}+7^{2019}\right)\)
\(6A=7^2+7^3+7^4+...+7^{2019}+7^{2020}-7^1-7^2-7^3-...-7^{2018}-7^{2019}\)
\(6A=-7^1+7^2-7^2+7^3-7^3+7^4-7^4+......+7^{2019}-7^{2019}+7^{2020}\)
\(6A=7^{2020}-7\)
\(A=\frac{7^{2020}-7}{6}\)
Câu b tương tự
