Question

Tìm x biết

a, ( x + 1 ) + ( x + 4 ) + ( x + 7 ) + ... + ( x + 28 ) = 155

b, 1 + \(\dfrac{1}{3}\) + \(\dfrac{1}{6}\) + \(\dfrac{1}{10}\) + ..... + \(\dfrac{2}{x+\left(x+1\right)}\) = \(1\dfrac{2009}{2010}\)

c, \(\left(\dfrac{2}{11.13}+\dfrac{2}{13.15}+\dfrac{2}{15.17}+.....+\dfrac{2}{19.21}\right)\) . 462 - x = 19

Answer 1

a, (x + 1) + (x + 4) + ... + (x + 28) = 155

x + 1 + x + 4 + ... + x + 28 = 155

(x + x + x + ... + x) + (1 + 4 + ... + 28) = 155

x . 10 + 145 = 155

x . 10 = 155 - 145

x . 10 = 10

x = 10 : 10

x = 1