tan[a+b]=(tan[a]+tan[b])/(1-tan[a]tan[b])=tan[45]=1
即1-tan[a]tan[b]=tan[a]+tan[b];
原式等于1+(tan[a]+tan[b])+tan[a]tan[b]=1+(1-tan[a]tan[b])+tan[a]tan[b]
=2