By observing the following formulas, we can find out what rules they have: 19 = 1 × 9 + (1 + 9); 29 = 2 × 9 + (2 + 9); 39 = 3 × 9 + (3 + 9) (1)1289= (2) The law is expressed as () urgent need

By observing the following formulas, we can find out what rules they have: 19 = 1 × 9 + (1 + 9); 29 = 2 × 9 + (2 + 9); 39 = 3 × 9 + (3 + 9) (1)1289= (2) The law is expressed as () urgent need

10n+9=9n+(n+9)

It is known that: 9 × 1 + 0 = 9, 9 × 2 + 1 = 19, 9 × 3 + 2 = 29, 9 × 4 + 3 = 39 According to the law of the former formula, write the sixth formula is______ ．

When n = 6, 9 × 6 + (6-1) = 59. So the answer is: 9 × 6 + (6-1) = 59

9×19×29×39×… What's the number of digits in 2009______ ．

According to the above analysis, the number of digits in this formula is 9, so the answer is: 9

The arithmetic sequence is 9, 19, 29, 39, ·····, find 409 in the number of terms

Because of AP
an=a1+(n-1)*d
d=10
a1=9
an=409
So 409 = 9 + (n-1) * 10
n=41