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`b)1+5+9+13+17+...+x=4950`

`(x+1)[(x-1):4+1]:2=4950`

`(x+1)[*(x-1)/4+4/4]=4950*2`

`(x+1)*(x-1+4)/4=9900`

`(x+1)*(x+3)=9900xx4`

`(x+1)(x+3)=39600`

`(x+1)(x+3)=198*200`

`(x+1)(x+3)=(197+1)(197+3)`

`=>x=197`

Vậy `x=197`

`c)(x-20)(1/2+1/3+...+1/200)/(1/199+2/198+...+198/2+199/2)=1/200`

`(x-20)(1/2+1/3+...+1/200)/(1/199+2/198+...+198/2+199)=1/200`

`(x-20)(1/2+1/3+...+1/200)/(1+1/199+1+2/198+...+1+198/2+1)=1/200`

`(x-20)(1/2+1/3+...+1/200)/(200/199+200/198+...+200/2+200/200)=1/200`

`(x-20)(1/2+1/3+...+1/200)/(200(1/200+1/198+...+1/2))=1/200`

`(x-20)*1/200=1/200`

`x-20=1`

`x=1+20`

`x=21`

Vậy `x=21`