for a, b, c being Element of F_Real
for M, N being Matrix of 3,F_Real st N = <*<*a,0,0*>,<*0,b,0*>,<*0,0,c*>*> holds
( ((M @) * (N * M)) * (1,1) = (((a * (M * (1,1))) * (M * (1,1))) + ((b * (M * (2,1))) * (M * (2,1)))) + ((c * (M * (3,1))) * (M * (3,1))) & ((M @) * (N * M)) * (1,2) = (((a * (M * (1,1))) * (M * (1,2))) + ((b * (M * (2,1))) * (M * (2,2)))) + ((c * (M * (3,1))) * (M * (3,2))) & ((M @) * (N * M)) * (1,3) = (((a * (M * (1,1))) * (M * (1,3))) + ((b * (M * (2,1))) * (M * (2,3)))) + ((c * (M * (3,1))) * (M * (3,3))) & ((M @) * (N * M)) * (2,1) = (((a * (M * (1,2))) * (M * (1,1))) + ((b * (M * (2,2))) * (M * (2,1)))) + ((c * (M * (3,2))) * (M * (3,1))) & ((M @) * (N * M)) * (2,2) = (((a * (M * (1,2))) * (M * (1,2))) + ((b * (M * (2,2))) * (M * (2,2)))) + ((c * (M * (3,2))) * (M * (3,2))) & ((M @) * (N * M)) * (2,3) = (((a * (M * (1,2))) * (M * (1,3))) + ((b * (M * (2,2))) * (M * (2,3)))) + ((c * (M * (3,2))) * (M * (3,3))) & ((M @) * (N * M)) * (3,1) = (((a * (M * (1,3))) * (M * (1,1))) + ((b * (M * (2,3))) * (M * (2,1)))) + ((c * (M * (3,3))) * (M * (3,1))) & ((M @) * (N * M)) * (3,2) = (((a * (M * (1,3))) * (M * (1,2))) + ((b * (M * (2,3))) * (M * (2,2)))) + ((c * (M * (3,3))) * (M * (3,2))) & ((M @) * (N * M)) * (3,3) = (((a * (M * (1,3))) * (M * (1,3))) + ((b * (M * (2,3))) * (M * (2,3)))) + ((c * (M * (3,3))) * (M * (3,3))) )