a ta có A + B + C + D = 360o
thay 100o+60o+90o+ D = 360o
=> D= 360o - ( 100o+60o+90o)
=> D=110o
b ta có A + B + C + D = 360
thay A + 45o+ 89o+130o = 360o
=> A = 360o- ( 45o+89o+130o)
=>A=96o
c, tac có A + B + C + D = 360o
thay A + 100o + 100o + 100o = 360o
=> A= 360o- (100o + 100o + 100o)
=> A = 60o