HELP SOLVING THESE TRIG. IDENTITIES PLEASE I BEG OF YOU BBY?

Make sure to show steps! Explain if possible!
9.) cos(x+y)cosy+sin(x+y)siny=cosx
10.) sin x- tan y cos x= (sin(x-y)/cos y)
12.) (tan(π/4 + x) - tan (π/4-x))/(tan(π/4+x) +tan (π/4-x)) = 2sinxcosx
13.) sin (x+y)sin(x=y)=cos²y-cos²x
14.) tan (x+y) tan (x-y)= (sin²x-sin²y)/(cos²x-sin²y)
Update: @michaelempeigne I meant (x-y) sorry mate, typo :(
2 answers 2