Im a bit stuck with both of these questions

Show that the function F(x, y, 2) = A cos INTHE ) COS (TV ) COS ("2 ) satisfies the equation(*) 22 F 22 F 02 F + dy2 + +B' F =0 (1) if TT B2 -+ -+ 12 where A, a, b and c are constants. (# Remark. You may like to know. Equation (1) appears in nuclear reactor theory where F : R3 - R called the neutron flux and B2 is a positive constant called buckling.Let u, I!) = 111(3u + 21.!) where u(a:, y] = 5:1:y and etc, 3;) = y2. To four decimal planes, use the Chain rule\") to nd % when I =1a.11d y =1. {*1 Remark. In this question, you are not allowed to directly substitute is end 1; in f to obtain e new function of :1: and y