True. If an angle is greater than 90° then the sum of the two others must be less than 90° which implies it is not a right angled triangle.
Work Step by Step
That is, suppose $A=100°\gt90°$. Then $B+C=80°$. So, $B\lt90°$ and $C\lt90°$ We have that $A\ne90°, B\ne90°,C\ne90°$