2x4分之一+4x6分之一+6x8分之一...+98x100分之一=

2x4分之一+4x6分之一+6x8分之一...+98x100分之一
=1/2×(1/2-1/4)+1/2×(1/4-1/6)+...+1/2×(1/98-1/100)
=1/2×(1/2-1/4+.+1/98-1/100)
=1/2×49/100
=49/200;再问一道：1/4+1/28+1/70+1/130+。。。+1/9700=1/4+1/28+1/70+1/130+。。。+1/9700

=1/3×(1-1/4)+1/3×(1/4-1/7)+....+1/3×(1/97-1/100)
=1/3×(1-1/4+1/4-1/7+...+1/97-1/100)
=1/3×(99/100)
=33/100;