$\"\"$

Observe the figure, So length of b = 40, hypotenuse c = 41.

To find the length a, Apply pythagorean theorem: $\"\"$ .

$\"\"$ (Substitute b = 40, and c = 41)

Evaluate powers: $\"\"$ .

$\"\"$ $\"\"$

Apply subtraction property of equality: a = b then $\"\"$ .

$\"\"$ (Subtract 1600 from each side)

$\"\"$ (Apply additive inverse property: $\"\"$ )

$\"\"$ (Subtract: $\"\"$ )

Apply square roots property: $\"\"$ then $\"\"$ .

$\"\"$

$\"\"$ (Evaluate power: $\"\"$ )

The positive value of a = 9. $\"\"$

The length of $\"\"$ .