高中数学数列、用裂项相消法解题、还有、裂项相消法怎么用、
求①sn=1/(2*5)+1/(3*6)+1/(4*7)+.+1/((n+1)(n+4))
②sn=1/(1*4)+1/(2*5)+1/(3*6)+.+1/(n*(n+3))