Question 1159788
The fifth term of an arithmetic progression is three times the second term,and the third term is 10.find the 20th term?
<pre>{{{matrix(1,3, a[2], "=", a[1] + d)}}}       {{{matrix(1,9, a[5], "=", a[1] + 4d, "=", 3(a[2]), "=", 3(a[1] + d), "=", 3a[1] + 3d)}}}    
                   {{{matrix(3,3, a[1] + 4d, "=", 3a[1] + 3d, 4d - 3d, "=", 3a[1] - a[1], d, "=", 2a[1])}}}
{{{matrix(1,5, a[2], "=", a[1] + 2d, "=", 10)}}} 
{{{matrix(1,5, a[2], "=", a[1] + 2(2a[1]), "=", 10)}}} ------ Substituting 2a<sub>1</sub> for d
      {{{matrix(3,3, a[1] + 4a[1], "=", 10, 5a[1], "=", 10, a[1], "=", 2)}}}
      {{{matrix(1,3, d, "=", 2a[1])}}}   
      {{{matrix(1,3, d, "=", 2(2))}}} ------ Substituting 2 for a<sub>1</sub>
      d, or common difference = 4  
{{{matrix(2,3, a[1], "=", 2, d, "=", 4)}}}, and so: {{{highlight_green(matrix(4,3, a[20], "=", a[1] + 19d, a[20], "=", 2 + 19(4), a[20], "=", 2 + 76, a[20], "=", highlight(78)))}}}