a
(4a+3c).(4b-3d)=(4b+3d).(4a-3c)
(4a+3c).4d-(4b+3c).3d=(4b+3d).4a-(4b+3d).3c
4a.4d+3c.4d-4b.3d+3c.3d=4b.4a+3d.4a-4b.3c+3d.3c
(đpcm)
b
(a+c)^2.(b-d)^2=(b+d)^2.(a-c)^2
(a+c)^2:(a-c)^2=(b+d)^2:(b-d)^2
(đpcm)
c
(2019a^3+2020b^3).(2019c^3-2020d^3)=(2019a^3-2020b^3).(2019c^3+2020d^3)
chuyển vế đổi dấu
(đpcm)