Question: how can I find index of something in a list?

A:=[X22(t) = 0, X26(t) = 0, X30(t) = 0, X34(t) = 0, X38(t) = 0, X42(t) = 0, X2(t) = X19(t), X5(t) = X23(t), X8(t) = X27(t), X11(t) = X31(t), X14(t) = X35(t), X17(t) = X39(t), X19(t) = X21(t), X23(t) = X25(t), X27(t) = X29(t), X31(t) = X33(t), X35(t) = X37(t), X39(t) = X41(t), X1(t) = Diff(X3(t), t), X4(t) = Diff(X6(t), t), X7(t) = Diff(X9(t), t), X10(t) = Diff(X12(t), t), X13(t) = Diff(X15(t), t), X16(t) = Diff(X18(t), t), X3(t) = Diff(X2(t), t), X6(t) = Diff(X5(t), t), X9(t) = Diff(X8(t), t), X12(t) = Diff(X11(t), t), X15(t) = Diff(X14(t), t), X18(t) = Diff(X17(t), t), -X20(t)-X22(t) = 0, -X24(t)-X26(t) = 0, -X28(t)-X30(t) = 0, -X32(t)-X34(t) = 0, -X36(t)-X38(t) = 0, -X40(t)-X42(t) = 0, X60(t) = X19(t), X62(t) = X23(t), X64(t) = X27(t), X66(t) = X31(t), X68(t) = X35(t), X70(t) = X39(t), X61(t) = X50(t), X61(t)+X20(t) = 0, X63(t) = X49(t), X65(t) = X51(t), X67(t) = X52(t), X63(t)+X24(t) = 0, X65(t)+X28(t) = 0, X67(t)+X32(t) = 0, X69(t)+X36(t) = 0, X71(t)+X40(t) = 0, X69(t) = X47(t), X71(t) = X48(t), X60(t) = X56(t), X62(t) = X54(t), X66(t) = X58(t), X70(t) = X53(t), X64(t) = X86(t), X68(t) = X88(t), Diff(X54(t), t) = X75(t), Diff(X55(t), t) = X77(t), Diff(X56(t), t) = X79(t), Diff(X57(t), t) = X81(t), Diff(X58(t), t) = X83(t), Diff(X59(t), t) = X85(t), Diff(X53(t), t) = X73(t), Diff(X75(t), t) = X74(t), Diff(X77(t), t) = X76(t), Diff(X79(t), t) = X78(t), Diff(X81(t), t) = X80(t), Diff(X83(t), t) = X82(t), Diff(X85(t), t) = X84(t), Diff(X73(t), t) = X72(t), Diff(X86(t), t) = X87(t), Diff(X88(t), t) = X89(t), sin(X86(t))+cos(X55(t))*cos(X58(t))*cos(X56(t))*sin(X54(t))+cos(X55(t))*cos(X58(t))*sin(X56(t))* cos(X54(t))-sin(X55(t))*cos(X58(t))*cos(X56(t))*cos(X54(t))+sin(X55(t))*cos(X58(t))*sin(X56(t))*sin(X54(t))+sin(X55(t))* sin(X58(t))*cos(X56(t))*sin(X54(t))+sin(X55(t))*sin(X58(t))*sin(X56(t))*cos(X54(t))+cos(X55(t))*sin(X58(t))*cos(X56(t))* cos(X54(t))-cos(X55(t))*sin(X58(t))*sin(X56(t))*sin(X54(t)) = 0]

I have A with 76 simple equations and 13 like last one which is big big with a lot of sin and cos.

How can I know what is index of big one which is 77?

I need to give this A to a proc or maple command and ask for index of first one which has a lot of sin and cos. I mean that one after Diff(X88(t), t) = X89(t).

Thanks