Pythagorean triplet adding up to 1000
for i in range(1,499): #max value for any side of the triangle must be less      than the semiperimeter
    for j in range(i,499):
        for k in range(j,499):
            if(i+j+k==1000 and i**2+j**2==k**2):



There are a couple quick optimizations.  

for i in range(1,333):


You can restrict this beyond 499.  We know that \$ a &lt; b &lt; c\$ and \$a + b + c = 1000\$, so we can say that \$3*a &lt; 1000\$ or \$a \leq 333\$.

    for j in range(i,499): #max value for any side of the triangle must be less than the semiperimeter
        k = 1000 - i - j


We don't need to iterate to find k.  We can calculate it from i and j since we know that the three sum to 1000.  

        if (i**2+j**2==k**2):


So we can just check that it's a Pythagorean triple.  The sum will always be correct, as that's how we select k.  

Just calculating k instead of iterating to find it should reduce the time to less than a second.