If 0<theta<90 and tan theta= (square root 3)/2, find cos theta?

tan(t) =�ã3 /2; cos(t) = 1/�ã(1+(tant)^2)= 1/�ã(1+3/4) = 2/�ã7

right or wrong?

You are doing this the hard way.

make a diagram
since tan theta = √3/2, and tan theta = opposite/adjacent, make the oppose √3 and the adjacent 2
In your head you could do Pythagoras and get the hypotenuse √7
since cos theta = adjacent/hypotenuse
cos theta = 2/√7

which is what you got.