Which of the following is an identity? A. secx tanx - cosx cotx = sinx B. (tanx + cotx) (sinx cosx) = 1 C. sin2x = 4 - 2cos2x D. cosx (2sinx + 1) = 0

Letter B

(tanx + cotx) (sinxcosx) = 1

[ (sinx/cosx) + (cosx/sinx) ] (sinxcosx) = 1

[ (sinxcosx) (sinx/cosx) + (sinxcosx) (cosx/sinx) ] = 1

[ (sinx) ^2 + (cosx) ^2] = 1

*Pythagorean Identity*

1 = 1