Đáp án:
$A=\sin\alpha.\cos\alpha=-\dfrac{8}{25}$
Giải thích các bước giải:
$\sin\alpha+\cos\alpha=\dfrac{3}{5}$
$⇒(\sin\alpha+\cos\alpha)^2=\dfrac{9}{25}$
$⇒\sin^2\alpha+2.\sin\alpha.\cos\alpha+\cos^2\alpha=\dfrac{9}{25}$
$⇒1+2.\sin\alpha.\cos\alpha=\dfrac{9}{25}$
$⇒2.\sin\alpha.\cos\alpha=-\dfrac{16}{25}$
$⇒\sin\alpha.\cos\alpha=-\dfrac{8}{25}$
Vậy $A=\sin\alpha.\cos\alpha=-\dfrac{8}{25}$.