Solution:

m tm = n tn

m [ a+ (m - 1) d ] = n [ a+ (n - 1) d ]

m a + m d (m – 1) = n a + n d (n – 1)

m a + m² d – m d = n a + n² d – n d

m a - n a + m² d - n² d – m d + n d = 0

a (m – n) + (m² - n² – m + n ) d = 0

a (m – n) + [ (m + n) (m - n) –( m - n ) ] d = 0

a (m – n) + (m – n) [ (m + n) – 1 ] d = 0

Divide by (m - n) => a + [ (m + n) – 1 ] d = 0

t (m + n) = 0

Therefore the (m+n) th term of the A.P is zero.