Hình như giải có câu `3` mà ạ ._.?

`(-9)/21 + 1/6 - 4/7 + 398/19 +10/12`

`=-3/7 + 1/6 - 4/7 + 398/19 + 5/6`

`=(-3/7-4/7)+(1/6+5/6)+398/19`

`=`\(\left(\dfrac{-3-4}{7}\right)+\left(\dfrac{1+5}{6}\right)+\dfrac{398}{19}\)

`=-7/7 + 6/6 + 398/19`

`=-1+1+398/19`

`=9+398/19`

`=398/19`

`C=3x^2 + 12x+13`

`C=3x^2 + 12x + 12 + 1`

`C=3.(x^3 +4x+4)+2`

`C=3.(x+2)^2 +1`

Mà : \(\left(x+2\right)^2\ge0\forall x\)

`=> 3.(x+2)^2 +1`\(\ge0\forall x\)

Vậy `C=3x^2 + 12x+13` luôn dương với mọi `x` \(\left(đfcm\right)\)

`(2/5-1/2)^2 - 2 1/5 : (-11)/15`

`=(4/10 - 5/10)^2 - 2 1/5 xx (-15)/11`

`=(-1/10)^2 - 2 1/5 xx (-15)/11`

`=1/100 - 11/5 xx (-15)/11`

\(=\dfrac{1}{100}-\dfrac{11.\left(-15\right)}{5.11}\)

`=1/100 - (-15)/5`

`=1/100 + 15/5`

`=1/100 + 3`

`=301/100`

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\(\dfrac{25^{45}.5^{43}}{125^{44}}\)

\(=\dfrac{5^{90}.5^{43}}{5^{132}}\)

\(=\dfrac{5^{90+43}}{5^{132}}\)

\(=\dfrac{5^{133}}{5^{132}}\)

\(=5^1\)

`=5`

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\(\sqrt{25}-\sqrt{49}+\sqrt{100}\)

`=5 - 7 + 10`

`=8`

`A=2^7 : 2^5 + 10`

`A = 2^(7-5) + 10`

`A=2^2 + 10`

`A=4+10`

`A=14`

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`B=3^2 . 3^2 - 3^2 . 4^3`

\(B=3^{2+2}-9.64\)

\(B=3^4-512\)

\(B=81-512\)

`B=-431`

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\(C=5^{2020}.5^{2020}+3^0+1\)

\(C=5^{2020+2020}+3^0+1\)

`C=5^4040 + 3^0 + 1`

`C=5^4040 + 1+1`

`C=5^4040 + 2`

`(1267-196)-(267+304)`

`=1267-196-267-304`

`=(1267-267)-(195+304)`

`=1000 - 500`

`=500`

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`(2002-79+15)-(-79+15)`

`=2002-79+15+79-15`

`=2002 +(-79+79)+(15-15)`

`=2002 + 0 + (15-15)`

`=2002 + 0+0`

`=2002`

Bn thay thử `x=13` vào biểu thức đó rồi xem thử có cùng kết quả ko nhé

`(x+1)+(x+3)+(x+5)+......+(x+19)=200`

`x+1+x+3+x+5+....+x+19=200`

`(x+x+x+....+x)+(1+3+5+.....+19)=200`

`=> 1+3+5+.....+19`

Số hạng:

`(19-1)/2 + 1 = 10(số-hạng)`

Tổng:

`(19+1)xx10:2=100`

`=> 10x + 100 = 200`

`10x=200-100`

`10x = 100`

`x=100:10`

`x=10`

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`102-40:(11-x)=82`

`40:(11-x)=102-82`

`40:(11-x)=20`

`11-x=40:20`

`11-x=2`

`x=11-2`

`x=9`

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`(x+2)+(x+4)+.....+(x+28)+(x+30)=315`

`x+2+x+4+.....+x+28+x+30=315`

`(x+x+......+x)+(2+4+.....+28+30)=315`

`=> 2+4+.....+28+30`

Số hạng:

`(30-2)/2 + 1 = 15(số-hạng)`

Tổng:

`(30+2)xx15:2=240`

`=> 15x + 240 = 315`

`15x=315-240`

`15x = 75`

`x=75:15`

`x=5`

`(x+3)/3 = 12/(x+3)`

`12.3 =(x+3).(x+3)`

`36 = (x+3)^2`

`6^2 = (x+3)^2`

`@TH1:`

`x+3=6`

`x=6-3`

`x=3`

`@TH2:`

`x+3=-6`

`x=-6-3`

`x=-9`

Vậy `x = {-9;3}`