Draw a cuboid with A 1, A2 and A3 as sides respectively.

A is the diagonal of a cuboid.

α, β and γ are angles formed by the diagonal line and three sides of the same vertex,

Cosα, cosβ and cosγ are the ratios of three sides to a.

So cos α = a1/a.

cosβ=a2/a，

cosγ=a3/a

a 1^2+a2^2+a3^2=a^2

So cos2α+cos2β+cos2γ = _ _1_ _