√1/2 + √1/3=
=√2/2+√3/3
=（3√2+2√3)/6
√64/9 X144/169
=√(8²×4²/13²)
=8×4/13
=32/13
√X² - 14X + 49=X - 7
√(X-7)²=|X-7|=X-7
X-7≥0
X≥7