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a) 125 . 101 = 125 . (100 + 1) = 125 . 100 + 125 . 1
= 12 500 + 125 = 12 625
b) 21 . 49 = 21 . (50 - 1) = 21 . 50 - 21 . 1
= 1 050 - 21 = 1 029
1. X thoả mãn ={-24;-23;-22;-21;-20;-19;-18;-17;-16;-15;-14;-13;-12;-11......;25}
=[(-24)+24]........+0+25
=25
2. a=3
1). Ta có: -25<x<26
\(\Rightarrow x\in\left\{-24,-23,-...,-1,0,1,2,...,25\right\}\)
Tổng các số nguyên x trên là :
-24 + ( -23 ) + ( -22 ) + ... + 25 =25
2) Ta có: \(7⋮2a+1\)
\(\Rightarrow2a+1\inƯ\left(7\right)=\left\{\pm1;\pm7\right\}\)
Ta có bảng sau :
| 2a+1 | 1 | -1 | 7 | -7 |
| a | 0 | -1 | 3 | -4 |
Vậy \(a\in\left\{0;-1;3;-4\right\}\)
A=1/2.3+1/3.4+1/4.5+...+1/99.100 ( Bạn nên viết như vậy để người khác dễ đọc hơn)
=1-1/2+1/2-1/3+1/3-1/4+...+1/99-1/100
=1-1/100
=99/100
#)Giải :
Cách 1 : \(A=\left\{9;11;13;15;17;19;21;23\right\}\)
Cách 2 : \(A=\left\{x\in N;x=2k+1;7< x< 25\right\}\)
một đống mặt
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\(\left(5^{56}+5^7\right):\left(5^{49}+1\right)=\left[5^7\left(5^{49}+1\right)\right]:\left(5^{49}+1\right)=5^7\)
a) 125.101
=125.(100+1)
=12625.
b)21.49
=21.(50-1)
=1029
# Aeri #
a. 125 . 101
= 125 . ( 100 + 1 )
= 125 . 100 + 125 . 1
= 12500 + 125
= 12 625