anonymous one year ago Which expression is equivalent to (sin x + 1)(sin x − 1)?

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1. anonymous

cos2x -cos2x cos2x + 1 cos2x − 1 -cos2x + 1

2. Nnesha

foil!!

3. Nnesha

|dw:1434123061043:dw| multiply

4. anonymous

D

5. P0sitr0n

First expand, then you can draw a conclusion: $sin^2x-1=(1-cos^2x)-1=-cos^2x$

6. Nnesha

nope how did you get that ?

7. P0sitr0n

also the expression is simply a difference of squares, so use the identity $(a-b)(a+b)=a^2-b^2$

8. anonymous

i am so confused if i foil i get D but you said no

9. P0sitr0n

$sin^2x+cos^2x=1$ use that

10. Nnesha

foil yes what did you get after this ??

11. anonymous

sin 2x -1

12. Nnesha

yes right now apply that identity sin^2 x + cos^2x =1 solve for sin^2x

13. Nnesha

$\large\rm sin^2 x = 1-cos^2$ trig identity agree or no?