I'm trying to prove this equality but I' stuck at the second step.
Please give me some hints or other ways to proceed.
tan2x+cos2xsinx+secx≡secx−sinxsinx=xcosx=yx2y2+y4y2xyy+1y≡1y−x=1−xyy(1)(x2+y4y2)(xy+1y)≡(2)x2+y4y(xy+1)≡(3)
Answer
The key is to see that (x+y)(x−y)=x2−y2.
tan2x+cos2xsinx+secx=tan2x+1−1+cos2xsinx+secx=sec2x−sin2xsinx+secx=secx−sinx.
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