2的平方—1分之一加3的平方—1分之一加4的平方减一分之一加.20的平方减一分之一
=1/(2-1)(2+1) +1/(3-1)(3+1)+1/(4-1)(4+1)...+1/(19-1)(19+1)+1/(20-1)(20+1)
=1/1*3 +1/2*4+1/3*5...+1/18*20+1/19*21
=1/2(2/1*3 +2/2*4+2/3*5...+2/18*20+2/19*21)
=1/2(1-1/3 +1/2-1/4+1/3-1/5...+1/18-1/20+1/19-1/21)
=1/2(1 +1/2-1/20-1/21)
=1/2*(589/420)
=589/840