(7+2t)^2乘(7-2t) =[(7+2t)^2]'乘(7-2t)+(7+2t)^2乘[(7-2t)]'
=2*(7+2t)*2*(7-2t)+(7+2t)^2*(-2)
=4*(49-4t^2)-2*(49+28t+4t^2)
=-24*t^2-56t+98
(7+2t)^2乘(7-2t) =[(7+2t)^2]'乘(7-2t)+(7+2t)^2乘[(7-2t)]'
=2*(7+2t)*2*(7-2t)+(7+2t)^2*(-2)
=4*(49-4t^2)-2*(49+28t+4t^2)
=-24*t^2-56t+98