p = bc+a
q = ab+c
r = ca+b
p + q + r = bc+a+ab+c+ca+b=2(a+b+c)2
=> p + q + r chan
+ neu p + q + r chan thi it nhat 2 trong 3 so do bang nhau
+ neu co 1 so = 2 thi gia su p = 2
<=> p = bc + a = 1 + 1
ma a,b,c nguyen duong => 2 = 1 + 1 = bc+a = ab+c
<=> p = q
=> dpcm
p=bc+a
q=ab+c
r=ca+b
quan he logic la bc+a voi ab+c va ca+b
tu do rut ra kl va dap an