 okay do this question number is this is given 3 6 12 blah blah blah SN is equal to what sorry S7 sum of first seven terms find out tell me 9 question number 10 simple application of formula formula is in your screen use GP is 1 1 by 2 1 by 4 1 by 8 dot dot dot S10 is equal to what 81 is correct a is 3 R is 2 SN is a RN minus 1 by R minus 1 so a is 2 so let me write this 3 into 2 to the power 7 minus 1 by 2 minus 1 simple formula no that's wrong S10 is wrong so hence it is 3 into 2 to the power 7 is 32 54 128 so 127 by 1 so 381 okay so here a is 1 R is half correct so S10 finite series a into R to the power n that is 10 minus 1 by 1 by 2 minus 1 correct so 1 by 2 yeah so it is 1 by 2 to the power 10 is 1024 minus 1 by minus 1 by 2 so which is yes so that means 1 minus 1024 by 1024 into 2 minus 2 so hence 1023 by 512 find the sum of 2 plus 6 plus 18 plus dot dot dot plus 4 3 7 4 what is SN a RN minus 1 by R minus 1 right so a RN minus a by R minus 1 right so a RN minus 1 into R minus a by R minus 1 am I right what is a R minus n minus 1 the last term so this is 4 3 7 4 into common ratio which is 3 minus first term which is 2 divided by 3 minus 1 that's it this answer is 6560 right so it didn't require to find out R but if you just in case you wanted to you can so 4 3 7 4 is first term 2 ratio 3 n minus 1 so this is 2187 is it so correct 217 is 3 to the power n minus 1 what is this 729 into 3 so this is 3 to the power 4 5 6 7 right so 7 is equal to n minus 1 so n is equal to 8 n was 8 and then you can find out some s8 is equal to 2 first term ratio 3 power 8 minus 1 divided by 3 minus 1 this was another method okay