Tesla has unveiled cheaper models of two of its most popular cars in the US as it tries to boost sales on the heels of the expiration of a key US tax credit. But the carmaker's shares tumbled about 4% as investors were underwhelmed by the announcement. In the US, the new versions of its Model Y mid-sized sport utility vehicle and Model 3 sedan are priced only $5,000 less than previous versions. Tesla, which faces growing competition, has lost ground as it has been slow to offer new, more