a) 2 + 3 × 2 mũ x = 26 b) ( 20 – x ) + 15 = 7 c) 45 : x; 60 : x; 75 : x; x > 5 d) tìm X thuộc N biết 8x

1 bình luận về “a) 2 + 3 × 2 mũ x = 26 b) ( 20 – x ) + 15 = 7 c) 45 : x; 60 : x; 75 : x; x > 5 d) tìm X thuộc N biết 8x”

1. a) 2+3*2^x=26

   3*2^x=26-2

   3*2^x=24

   2^x=24:3

   2^x=8

   2^x=2^3

   =>x=3

   Vậy x=3

   b)(20-x)+15=7

   20-x=7-15

   20-x=-8

   -x=-8-20

   -x=-28

   x=-28

   Vậy x=-28

   c)45 vdots x; 60 vdots x; 75 vdots x

   =>x in ƯC(45;60;75)

   Ta có:

   {(60=2^2*3*5), (45=3^2*5), (75=3*5^2):}

   =>ƯCLN(45;60;45)=3*5=15

   =>x in ƯC(45;60;75)=Ư(15)={+-1; +-3; +-5;+-15}

   Vì x>5 nên x=15

   Vậy x=15

   d) Ta có: 8x+30 vdots 2x+3

   <=>8x+12+18 vdots 2x+3

   <=>(8x+12)+18 vdots 2x+3

   <=>4(2x+3)+18 vdots 2x+3

   Do 4(2x+3) vdots 2x+3

   Nên 18 vdots 2x+3

   =>2x+3 in Ư(18)={+-1; +-2; +-3; +-6; +-9; +-18}

   =>2x in {-4;-2; -1; -5;-6; 0;-9; 3; -12; 6; -21; 15}

   =>x in {-2; -1; -1/2; -5/2; -3;-9/2; 0; 3/2; -6; 3; -21/2; 15/2}

   Vậy x in {-2; -1; -1/2; -5/2; -3;-9/2; 0; 3/2; -6; 3; -21/2; 15/2}  

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