There are two types of triangular matrices.

**1.** Upper Triangular Matrix: A square matrix (a_{ij})
is said to be an upper triangular matrix if all the elements below the principal
diagonal are zero (0). That is, [a_{ij}]_{m} _{× n} is an
upper triangular matrix if (i) m = n and (ii) a_{ij} = 0 for i > j.

Examples of an Upper Triangular Matrix are:

(i) \(\begin{bmatrix} 5 & 2 & 8\\ 0 & 3 & 10\\ 0 & 0 & 8 \end{bmatrix}\)

(ii) \(\begin{bmatrix} -1 & 7 & 3\\ 0 & 6 & 1\\ 0 & 0 & 5 \end{bmatrix}\)

(iii) \(\begin{bmatrix} 3 & 0 & 3\\ 0 & 7 & -1\\ 0 & 0 & 2 \end{bmatrix}\)

2. Lower Triangular Matrix: A square matrix (a_{ij})
is said to be a lower triangular matrix if all the elements above the principal
diagonal are zero (0). That is, [a_{ij}]_{m} _{× n} is a
lower triangular matrix if (i) m = n and (ii) a_{ij} = 0 for i < j.

Examples of a Lower Triangular Matrix are:

(i) \(\begin{bmatrix} 7 & 0 & 0\\ 3 & 9 & 0\\ 1 & 2 & 1 \end{bmatrix}\)

(ii) \(\begin{bmatrix} 1 & 0 & 0\\ -5 & 1 & 0\\ 3 & 7 & 1 \end{bmatrix}\)

(iii) \(\begin{bmatrix} 9 & 0 & 0\\ 1 & 3 & 0\\ 2 & 5 & -4 \end{bmatrix}\)

**Definition of Triangular
Matrix:**

A square matrix is said to be a triangular matrix if it is either upper triangular or lower triangular.

For example:

(i) \(\begin{bmatrix} 2 & 3 & 1\\ 0 & 1 & 3\\ 0 & 0 & 4 \end{bmatrix}\)

(ii) \(\begin{bmatrix} 1 & 0 & 0\\ 2 & 3 & 0\\ 4 & 1 & 2 \end{bmatrix}\)

(iii) \(\begin{bmatrix} 0 & 0 & 0\\ 3 & 0 & 0\\ 2 & 1 & 0 \end{bmatrix}\)

(iv) \(\begin{bmatrix} 0 & 1 & 2\\ 0 & 0 & 3\\ 0 & 0 & 0 \end{bmatrix}\)

A diagonal matrix is both upper triangular and lower triangular.

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