Lơ giải:
\(\frac{25}{16}=(\sin a+\cos a)^2=\sin ^2a+\cos ^2a+2\sin a\cos a=1+2\sin a\cos a\)
\(\Rightarrow \sin a\cos a=\frac{9}{32}\)
\((\sin a-\cos a)^2=(\sin a+\cos a)^2-4\sin a\cos a=\frac{25}{16}-4.\frac{9}{32}=\frac{7}{16}\)
\(\Rightarrow \sin a-\cos a=\pm \frac{\sqrt{7}}{4}\)
Do đó:
\(D=\sin ^3a-\cos ^3a=(\sin a-\cos a)(\sin ^2a+\sin a\cos a+\cos ^2a)\)
\(=(\sin a-\cos a)(1+\sin a\cos a)\)
\(=\pm \frac{\sqrt{7}}{4}(1+\frac{9}{32})=\pm \frac{41\sqrt{7}}{128}\)
Lơ giải:
\(\frac{25}{16}=(\sin a+\cos a)^2=\sin ^2a+\cos ^2a+2\sin a\cos a=1+2\sin a\cos a\)
\(\Rightarrow \sin a\cos a=\frac{9}{32}\)
\((\sin a-\cos a)^2=(\sin a+\cos a)^2-4\sin a\cos a=\frac{25}{16}-4.\frac{9}{32}=\frac{7}{16}\)
\(\Rightarrow \sin a-\cos a=\pm \frac{\sqrt{7}}{4}\)
Do đó:
\(D=\sin ^3a-\cos ^3a=(\sin a-\cos a)(\sin ^2a+\sin a\cos a+\cos ^2a)\)
\(=(\sin a-\cos a)(1+\sin a\cos a)\)
\(=\pm \frac{\sqrt{7}}{4}(1+\frac{9}{32})=\pm \frac{41\sqrt{7}}{128}\)
