If is the pull applied at the ends of tape in kg, is the length of tape between end marks in
metres, is the weight of the tape in kg per metre run, then sag correction
(A) C = w²l3 /24F²
(B) C = w3l²/24F²
(C) C = w²l3 /24F3
(D) C = 24w²l3 /80F3