Question

tìm x

a,x²+5x=0

b,1/21+1/28+1/36+...+2/x(x+1)=2/9

(giúp mk nha mk cần gấp)

Answer 1

a) x² + 5x = 0

=> x.(x + 5) = 0

=> x = 0 hoặc x + 5 = 0

=> x = 0 hoặc x = -5

b) 1/21 + 1/28 + 1/36 + ... + 2/x(x + 1) = 2/9

=> 2/42 + 2/56 + 2/72 + ... + 2/x(x + 1) = 2/9

=> 2 × [1/6×7 + 1/7×8 + 1/8×9 + ... + 1/x(x + 1)] = 2/9

=> 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + ... + 1/x - 1/x + 1 = 2/9 : 2

=> 1/6 - 1/x + 1 = 2/9 × 1/2 = 1/9

=> 1/x + 1 = 1/6 - 1/9

=> 1/x + 1 = 3/18 - 2/18

=> 1/x + 1 = 1/18

=> x + 1 = 18

=> x = 17

Answer 2

Tìm x

b)  x²+2x+1=0

help me