30*29-29*28+28*27-27*26+…+4*3-3*2
=(30*29-29*28) + (28 * 27-27*26) + .+ ((4*3-3*2)
=29*2 + 27*2 + 25*2 + .+ 3*2
=2* (29+27+25+...+3)
=2* (3+29) * 14 /2
=32 * 14
=448