Atari 400 killed Pythagoras' Theorem (Score 0)

Yet another precision problem from the old days, shaken the very foundatation of my math teacher, he can't figure out what's wrong and he refused to accept what he's seen.

First part was the actual session, second part was my debug session

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LIST
10 INPUT "1st side", A
20 INPUT "2nd side", B
30 INPUT "3rd side", C
40 E = SQRT(A*A + B*B)
50 PRINT "This is ";
60 IF E C THEN PRINT "not";
70 PRINT " a right angled triangle"
80 END
OK

RUN
1st side
3
2nd side
4
3rd side
5
This is not a right angled triangle.
OK

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PRINT SQRT(25)
5
OK

PRINT (SQRT(25) - 5) * 10000
0.00001
OK

Damn!
SYNTAX ERROR