The length of a diagonal of the square is 21cm.
A square can have two diagonals. Each of the diagonal can be formed by joining the diagonally opposite vertices of a square.
A square has a side length of 15 cm.
The following is closest to the length of a diagonal of the square is;
[tex]\rm d^2 =15^+15^2\\\\d^2=225+225\\\\d^2=450\\\\d=21[/tex]
Hence, the length of a diagonal of the square is 21cm.
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