00:01
So in the given question we are told that a, b and c are the lengths of the lengths of sides of a cuboid, right? it is the length of sides of a cuboid and the cuboid is having a unit volume, right? it is the length of sides of a cuboid of unit volume.
00:35
So we have a matrix in the question that is given as a, b, c, b, c, a, c, a, b, c, a, b, c, a, b, and we are told that this matrix is an involuntary matrix.
00:53
And if these are given then we are told that we should find the value for a cube plus b plus c cube so this is what we have in the question so what we can do over here is let's take the matrix a first right it is said to be involuntary right which means when we take the square of a what we would get is the identity matrix, right? so then we can write when we take the determinant of a, we would get determinant of a squared as equal to 1 since determinant of i is equal to 1.
01:45
So determinant of a squared is equal to 1 which means determinant of a is equal to plus or minus 1, right we can write this as determinant of determinant of a b c b c a c a b c a b is equal to plus or minus 1 and from this let's expand this determinant right so what you would have is a times b c minus a square minus b times b square minus ac and we have plus c times ab minus c square and this is equal to plus or minus 1 is equal to plus or minus 1 right so now when we expand this what we would have is a b c minus a cube minus b cube minus plus a b c plus a b c is equal to plus or minus one from which we can write three a bc minus b cube minus minus b cube minus c is equal to plus or minus one and from here what we can do is we can take the value of a cube plus b cube plus c cube as 3a b b c plus or minus 1...