（1/3-7/12+9/20-11/30+13/42-15/56)x2^3x21

（1/3-7/12+9/20-11/30+13/42-15/56)x2^3x21
=[1/3-(1/3+1/1/4)+(1/4+1/5)-(1/5+1/6)+(1/6+1/7)-(1/7+1/8)]x2^3x21
=(-1/8)x2^3x21
=-21

（1/3-7/12+9/20-11/30+13/42-15/56)x2^3x21
=（1/3-（1/3+1/4) +(1/4+1/5)-(1/5+1/6)+(1/6+1/7)-(1/7+1/8) )x2^3x21
=(1/3-1/3-1/4 +1/4+1/5-1/5-1/6+1/6+1/7-1/7-1/8) )x2^3x21
=-1/8x8x21
=-21