let n be Nat; :: thesis: for x1, x2, x3 being Element of REAL n holds x1 + (x2 - x3) = (x1 + x2) - x3
let x1, x2, x3 be Element of REAL n; :: thesis: x1 + (x2 - x3) = (x1 + x2) - x3
thus x1 + (x2 - x3) = (x1 + x2) + (- x3) by RVSUM_1:15
.= (x1 + x2) - x3 ; :: thesis: verum