t属于(0,π),sint+cost=1/3,求cos2t

问题描述:

t属于(0,π),sint+cost=1/3,求cos2t

∵(sint+cost)^2
=1+2sintcost
=1/9
∴sintcost=-4/9
∵t∈(0,π)
∴sint>0
∵sintcost