解答题 21.设z=(x²+y²)^sec²(x+y)，求

【正确答案】由z=(x²+y²)^sec²(x+y)，得z=e^{sec²(x+y)ln(x²+y²)}，
则

=e^{sec²(x+y)ln(x²+y²)}[2sec²(x+y)tan(x+y)ln(x²+y²)+

sec²(x+y)]，

=e^{sec²(x+y)ln(x²+y²)}[2sec²(x+y)tan(x+y)ln(x²+y²)+

【答案解析】