Trigonometric Identity 4 Leaving Certificate Higher Level Maths

Trigonometric Identity 4 Leaving Certificate Higher Level Maths

[OfSoundNFury]

is there anything preventing me doing this? :
cos(A-B) = cosAcosB + SinAsinB
cos(A-A) = cosAcosA + SinAsinA
cos(0) = cos^2(A) + sin^2(A)
1 = coa^2(A) + sin^2(A)
1 = 1 (pg 16 log tables)