a) `sin^2 alpha+ cos^2 alpha=1 => (sin^2 alpha+ cos^2 alpha)^2=1 =>` Đúng

b) `(sin alpha+cos alpha)^2=sin^2 alpha+cos^2 alpha+2 sin alpha * cos alpha=1+2 sin alpha*cos alpha`

`=>` Sai

c) `(sin alphac-cos alpha)^2=sin^2 alpha+cos^2 alpha-2 sin alpha * cos alpha=1-2 sin alpha*cos alpha`

`=>` Sai

d) `sin^4 alpha+cos^4 alpha=(sin^2 alpha+cos^2 alpha)-2 sin^2 alpha. cos^2 alpha=1-2 sin^2 alpha* cos^2 alpha`

`=>` Sai