Prove the cofunction identity using the addition and subtrac

Prove the cofunction identity using the addition and subtraction formulas:

Prove the identity:

cos(13pi/15)*cos(-pi/5) - sin(13pi/15)*sin(-pi/5)

use the cos(A +B) = cosAcosB - sinAsinB

So,A = 13pi/15 : B = -pi/5

So, cos(13pi/15)*cos(-pi/5) - sin(13pi/15)*sin(-pi/5) = cos( 13pi/15 -pi/5)

= cos( 10pi/5) = cos2pi =1

2) csc(pi/2 - u) = secu

we know trigonometric identity: sin(pi/2 -x) = sinpi/2cosx - cospi/2sinx = cosx

So, 1/sin(pi/2 -x) = 1/cosx

csc(pi/2 -x) = secx

3) 1 -tanx*tany = cos(x+y)/cosx*cosy

LHS :

1 -tanx*tany = 1 -sinx/cosx*siny/cosy

= (cosxcosy -sinxsiny)/cosxcosy

= cos(x+y)/cosxcosy

= RHs