let G, F be AbGroup; :: thesis:
thus for x being set holds
( x is Element of [:G,F:] iff ex x1 being Element of G ex x2 being Element of F st x = [x1,x2] ) by SUBSET_1:43; :: thesis:
thus for x, y being Element of [:G,F:]
for x1, y1 being Element of G
for x2, y2 being Element of F st x = [x1,x2] & y = [y1,y2] holds
x + y = [(x1 + y1),(x2 + y2)] by PRVECT_3:def 1; :: thesis:
thus 0. [:G,F:] = [(0. G),(0. F)] ; :: thesis:
thus for x being Element of [:G,F:]
for x1 being Element of G
for x2 being Element of F st x = [x1,x2] holds
- x = [(- x1),(- x2)] :: thesis:

proof

let x be Element of [:G,F:]; :: thesis:
let x1 be Element of G; :: thesis:
let x2 be Element of F; :: thesis:
assume A1: x = [x1,x2] ; :: thesis:
reconsider y = [(- x1),(- x2)] as Element of [:G,F:] ;
x + y = [(x1 + (- x1)),(x2 + (- x2))] by A1, PRVECT_3:def 1
.= [(0. G),(x2 + (- x2))] by RLVECT_1:def 10
.= 0. [:G,F:] by RLVECT_1:def 10 ;
hence - x = [(- x1),(- x2)] by RLVECT_1:def 10; :: thesis:

end;