[+] × [+] = [+] [−] × [−] = [+] [−] × [+] = [−] [+] × [−] = [−] Kuis ( + ) + ( + ) = 1 + 1 + 2 + 7 = 2 + ( + ) ( + ) − = 1 + 1 − 3 − 8 = −2 − ( − ) ( − ) + = − 2 ( − ) ( − ) − = + ( ) ( + ) + = + ( ) ( − ) = − +