típ nha m.n
18) (2+x ) +(4+x)+(6+x )+...+(52+x) = 780
(1/3.5 +1/5.7+1/7.9 +...+1/19.21).x = 9/7
20) (x+1)+(x+2)+...+(x+100)=5750
22) (x+1/1.3)+(x.1/3.5)+(x+1/5.7)+...+(x+23.25)= 11.x+(1/3+1/9+1/27+1/81+1/243 )
23) (1/1.2.3 +1/2.3.4+....+1/8.9.10).x= 23/45
\(\sqrt{ }\) | |||||||