17.
From the question:
if x not equal to 0
Precoditions:
(1)
if x>0
(2)
if x<0 (since x not equal ot zero dont take x as 0)
Post condition:
{y>0}
case1:
if x>0 then
let x = 1
y= 2*x = 2*1 = 2
since 2>0 the post condition y>0 is true.
case2:
else // it indicatesif x<0
let x = -3
y= (-2)*x = 2*-(-3) = 6
since 6>0 the post condition y>0 is true.
Hence proved
18.
From the question:
if x not equal to 0
Precoditions:
(1)
if x>0 (since x not equal ot zero dont take x as 0)
(2)
if x<0
Post condition:
result is |x|
case1:
if x>0 then
let x=1
abs = x = 1
since x = abs = |x| = 1 the post condition |x| is
true.
case2:
else // it indicatesif x<0
let x=-1
abs = -x = -(-1) = 1
since x = -1 and abs = |x| = 1 the post condition |x| is true.
Hence proved
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