How can i convert sin(theta) into cos(theta)?

Just use the following formula: 

> sin^2(theta) + cos^2(theta) = 1 

So, for " cos(theta) = 6/(√60) " therefore: " cos^2(theta) = [6/(√60)]^2 = 36/60 " 

Now. sin^2(theta) + cos^2(theta) = 1 

So, sin^2(theta) + 36/60 = 1 

So, sin^2(theta) = 1 - 36/60 = 60/60 - 36/60 = 24/60 = 2/5 

So, sin^2(theta) = 2/5 

Therefore: Either: >>>>> sin(theta) = +√(2/5) 

Or: >>>>> sin(theta) = -√(2/5) 

Hope I helped. :)