If the side length of regular triangle ABC is a, PA ⊥ plane ABC, PA = AB, then the distance from a to plane PBC is

Root of seven twenty one a
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1. If the distance between point (4, a) and line 4x-3y-1 = 0 is not greater than 3, then the value range of a is () A. [0，10]B. [13，313]C. （0，10）D. （-∞，0]∪[10，+∞）
2. The fifth power of polynomial ax + the third power of BX + (X-5) when x = 3, the value is equal to 7. When x = - 3, find the value of this polynomial fast The process is detailed ah
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