If I ( r) `= r(r^(2)-1) ` then find `sum_(r=2)^(oo) (1)/(I( r))`.

If sum_(r=1)^(n) T_(r) = n(2n^(2)+9n +13) then find sum_(r=1)^(n) sqrt(T_(r)) .

Find the sum sum _(r=1)^(n) (r )/((r+1)!)

Find the value of sum_(r=0)^(oo)tan^(-1)(1/(1+r+r^2))

If 5 xx 4P_r = 6 xx 5P_(r-1) find 'r'.

Find the value of sum_(r=1)^(n-1)"sin"^(2)(rpi)/(n)

If z _(r) = cos ((pi)/(2 ^(r))) + i sin ((pi )/( 2 ^(r))), then z _(1) . z _(2). z _(3)... upto oo equals : a)-3 b)-2 c)-1 d)0

Find r if 5P_r =6p_(r-1)