Let's first simplify the expression by using the difference of squares formula:

(cos215° - sin215°)·(cos215° + sin215°)
= cos^2(215°) - sin^2(215°)

Next, we can use the trigonometric identities to rewrite cos^2(215°) and sin^2(215°):

cos(2θ) = cos^2(θ) - sin^2(θ)
sin(2θ) = 2sin(θ)cos(θ)

Thus, we have:

cos(2(215°)) = cos(430°)
sin(2(215°)) = sin(430°)

Since the cosine function is periodic with a period of 360 degrees, we can simplify further:

cos(430°) = cos(430° - 360°) = cos(70°)
sin(430°) = sin(430° - 360°) = sin(70°)

Therefore, the expression simplifies to:

cos^2(215°) - sin^2(215°)
= cos(70°) - sin(70°)

Therefore, (cos215° - sin215°)·(cos215° + sin215°) simplifies to cos(70°) - sin(70°).