4cos^2alpha – 3sin^2alpha với cos alpha =4/7 08/08/2021 Bởi Claire 4cos^2alpha – 3sin^2alpha với cos alpha =4/7
`4\cos^2\alpha-3\sin^2\alpha` `=4\cos^2\alpha-3+3\cos^2\alpha` `=7\cos^2\alpha-3` $(*)$ Thay `cos\alpha=4/7` vào $(*)$ `=7.(\frac{4}{7})^2-3` `=16/7-3` `=[-5]/7` Bình luận
$4\cos^2\alpha-3\sin^2\alpha$ $=4\cos^2\alpha-3+3\cos^2\alpha$ $=7\cos^2\alpha-3$ $=7.\Big(\dfrac{4}{7}\Big)^2-3$ $=\dfrac{-5}{7}$ Bình luận
`4\cos^2\alpha-3\sin^2\alpha`
`=4\cos^2\alpha-3+3\cos^2\alpha`
`=7\cos^2\alpha-3` $(*)$
Thay `cos\alpha=4/7` vào $(*)$
`=7.(\frac{4}{7})^2-3`
`=16/7-3`
`=[-5]/7`
$4\cos^2\alpha-3\sin^2\alpha$
$=4\cos^2\alpha-3+3\cos^2\alpha$
$=7\cos^2\alpha-3$
$=7.\Big(\dfrac{4}{7}\Big)^2-3$
$=\dfrac{-5}{7}$