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2 . tìm x :
a ) 52x = 625
52x = 54
=> 2 . x = 4
x = 4 : 2
x = 2
b ) 9x-1 = 9
9x-1 = 91
=> x - 1 = 1
x = 1 + 1
x = 2
c ) 2x : 25 = 1
2x-5 = 1
=> x = 5 , vì 25-5 = 20 = 1
S1 = 10 + 12 + 14 + ... + 2010: Có 1001 số hạng
=> (10 + 2010) . 1001 : 2 = 1011010
S2 = 21 + 23 + 25 + ... + 1001: Có 491 số hạng
=> (21 + 1001) . 491 : 2 = 250901
S3 = 1 + 4 + 7 + ... + 79: Có 27 số hạng
=> (1 + 79) . 27 : 2 = 1080
S4 = 15 + 17 + 19 + 21 + ... + 151 + 153 + 155: Có 71 số hạng.
=> (155 + 15) . 71 : 2 = 6035
a) \(S_1=1+2+3+4+......+999\)
\(\Rightarrow S_1=\dfrac{\left(999+1\right).\left[\left(999-1\right):1+1\right]}{2}\)
\(\Rightarrow S_1=\dfrac{1000.\left(998+1\right)}{2}\)
\(\Rightarrow S_1=\dfrac{1000.999}{2}\)
\(\Rightarrow S_1=\dfrac{999000}{2}\)
\(\Rightarrow S_1=499500\)
b) \(S_2=10+12+14+......+2010\)
\(\Rightarrow S_2=\dfrac{\left(2010+10\right).\left[\left(2010-10\right):2+1\right]}{2}\)
\(\Rightarrow S_2=\dfrac{2020.\left(2000:2+1\right)}{2}\)
\(\Rightarrow S_2=\dfrac{2020.\left(1000+1\right)}{2}\)
\(\Rightarrow S_2=\dfrac{2020.1001}{2}\)
\(\Rightarrow S_2=\dfrac{2022020}{2}\)
\(\Rightarrow S_2=1011010\)
c) \(S_3=21+23+25+.......1001\)
\(\Rightarrow S_3=\dfrac{\left(1001+21\right).\left[\left(1001-21\right):2+1\right]}{2}\)
\(\Rightarrow S_3=\dfrac{1022.\left(980:2+1\right)}{2}\)
\(\Rightarrow S_3=\dfrac{1022.\left(490+1\right)}{2}\)
\(\Rightarrow S_3=\dfrac{1022.491}{2}\)
\(\Rightarrow S_3=\dfrac{501802}{2}\)
\(\Rightarrow S_3=250901\)
d) \(S_5=1+4+7+......+79\)
\(\Rightarrow S_5=\dfrac{\left(79+1\right).\left[\left(79-1\right):3+1\right]}{2}\)
\(\Rightarrow S_5=\dfrac{80.\left(78:3+1\right)}{2}\)
\(\Rightarrow S_5=\dfrac{80.\left(26+1\right)}{2}\)
\(\Rightarrow S_5=\dfrac{80.27}{2}\)
\(\Rightarrow S_5=\dfrac{2160}{2}\)
\(\Rightarrow S_5=1080\)
e) \(S_7=15+25+35+45+......+115\)
\(\Rightarrow S_7=\dfrac{\left(115+15\right).\left[\left(115-15\right):10+1\right]}{2}\)
\(\Rightarrow S_7=\dfrac{130.\left(100:10+1\right)}{2}\)
\(\Rightarrow S_7=\dfrac{130.\left(10+1\right)}{2}\)
\(\Rightarrow S_7=\dfrac{130.11}{2}\)
\(\Rightarrow S_7=\dfrac{1430}{2}\)
\(\Rightarrow S_7=715\)
\(s1=1+2+3+4+5+6+.....+999\)
\(s1=\left(1+999\right)+\left(2+998\right)+.....+\left(499+501\right)+500\)có 499 cặp
\(s1=1000+1000+.....+1000\)có 499 số 1000\(+500\)
\(s1=1000\times499+500\)
\(s1=499000+500\)
\(s1=499500\)
\(s2=21+23+25+....+1001\)
\(s2=\left(1001-21\right):2+1=491\)
\(s2=\left(1001+21\right)\times491:2\)
\(s2=1022\times491:2\)
\(s2=501802:2\)
\(s2=250901\)
1)
S1=(1+999)+(2+998)+...+(501+499)+500
S1=1000.(999-1)+500
S1=998 000 + 500
S1=998 500
:/ câu 1 quá ez mik bik làm rồi còn các câu sau nó thì mik chịu