In ∆ABC, ∠B = 90° , and l(BC) = 21, and l(AB) = 20

∴ According to Pythagoras’ theorem,

∴ l(AC)^{2 }= l(BC)^{2 }+ l(AB)^{2 }

∴ l(AC)^{2 }= 21^{2 }+ 20^{2 }

∴ l(AC)^{2 }= 441 + 400

∴ l(AC)^{2 }= 841

∴ l(AC)^{2 }= 29^{2 }

∴ l(AC) = 29

Perimeter of ∆ABC = l(AB) + l(BC) + l(AC)

= 20 + 21 + 29

= 70

∴ The length of hypotenuse AC is 29 units, and the perimeter of ∆ABC is 70 units.