1. Let the first number of the first four digits be X, then x+(x- 1)+(x-2)+(x-3)=26, so the first four digits are 5432.

Decompose 336 into prime factors

336=3×7×2×2×2×2 Because 2 3 7 is a prime number, 2× 2 = 8 is just a sum number, so the last four digits are 2378.

So the phone number is 54322378.

2.3/5+5/203/6+6/203/7+7/203/8+8/203/9+203/ 10+ 10/20 = 368+8/20.

3. Solve n = 67 1+(n-1) x3 = 2011at 671.

This is a series of arithmetic progression, the nth term An = a 1+(N- 1) D 'an is the nth term A1is the first term, n is the number, and d is arithmetic.

I don't know if you understand.

The sum of the numbers before 10 is14510x (1+28)/2 =145, and the formula is also sn=n(a 1+an)/2.

Ask me if you don't understand!

The third question is the sixth grade? I suspect that if it is, this formula will be regarded as a problem of finding laws.